Why bother saying anything about it now with it having been discontinued? Well if anything it could be used as a benchmark and a guidelines of what you should expect (and beyond) for all subsequent iterations as well as the most current model, and after 5 years in its multiple roles as a kind of workhorse for the main part, its proven itself to be more than reliable despite a few gripes about performance which weren't necessarily down to the local machine getting any worse as such despite seeming demands elsewhere changing depending on which OS I was using and which country I was in. At the time of preview and general release the M6400 was a machine that covered much of my wishlist for a laptop at the time.

More desktop like computing power on vastly less electrical power consumption

From the outset there was something very special about the concept of the Precision M6400 when it was new 6 years ago. Sure there were 17" DELL precisions machines prior to the M6400, however there seemed to be an unofficial push and an air of it being designed to be servicable by the end user or enthusiast with the will to do so. For example many of the most commonly used interface ports (potentially most commonly used depending on what you used it for) were modular in the sense that if they broke, you could change them just a single part without having to replace the entire mainboard (which was at the time quite expensive). This also applied to many of the other components such as the sound card, GPU, card reader, firewire ports, USB ports, E-SATA ports, Processor, RAM... in fact everything. This obviously also helps with the general scope for upgradability of suitably compatible parts, it was pretty much like a regular desktop PC in that sense but in 17 inch laptop form.

Essentially the M6400 was designed as a desktop machine replacement, and although it was portable just like a laptop, using it as laptop isn't the most ideal for general day to day portable use (which I've done). But the laptop form factor obviously makes the task of moving it as a main machine much easier than a tower style desktop.

Computing Power

There's not really much point talking about the M6400 processing power, mainly because corei processors are the now the main staple of number crunchers as the standard for these machines from the M6500, M6600 and M6700 onwards. However it was potentially the most powerful machine of its type within its generation at a time when quadcore mobile computing of the desktop variety was nearly unthinkable on the general consumer domain. The main thing was that base Precision m6400 design was scalable according to budget without having to change to an entirely different machine as your needs required. you could start off a with a paltry P8200 and work your way up to anything in between before getting at the top end QX9300 quadcore under the hood. At the point of release a potential maximum 16GB of ram for general desktop machines was hardly common either, let alone a laptop based machine. However I will say this, the performance of the machine did seem to fluctuate depending on which country I was in, Windows 7 generally seemed to perform much more quickly and was generally snappier whilst I was living in SE Asia, it became even more pronounced with test builds of Windows 8. But generally whilst in Europe the machine felt a lot more sluggish under Windows 7 and eventually test builds of Windows 8 too. Ontop of this my machine was scoring lower and lower year upon year on the performance index rating despite the a fore mentioned anomalous difference in performance depending on OS and Geo location. This is also having upgraded the machine with more ram, a hybrid SSD and a full SSD as well as a faster processor. (This was also with no real increase in resource requirements for what I was using the machine for).

Generally though with all the more recent i5 and i7 variants (post M6600) I reckon there isn't going to be nearly the same kinds of inconsistencies as there were with the early core2duo and quadcore variant. 

Upgrades its had and Hardware issues I'd encountered with the machine in 5 years of use...

- Moved over from a standard LCD screen to an LED screen when the screen accidentally got cracked - The difference in picture quality and colour depth is stark and is vastly more impressive on the true LED screen. The LED panel also doesn't require and inverter and also used less electrical power. The LED screen requires a different internal ribbon cable from the standard LCD screen, and unfortunately the second hand replacement internal LED screen cable that I'd bought for it had a damaged webcam cable plug. The webacm cable being integrated into the the LED cable as part of it. So I've not had a functioning built in webcam since.

- Moved over from a standard 320GB 16MB 7200RPM drive to a 500GB Seagate hybrid SSD (faster then a 3.5" 10K raptor). despite this Ableton seemed to gradually perform worse over time despite the rest of my set up not being anymore resource intensive. Obviously something going on at the server side of things away from my local system.

- Fingerprint reader was somehow damaged (the connecting ribbon cable). thus was subsequently replaced. I guess it was designed under the premise that people wouldn't really need to lift open the upper part of the bezel/speaker strip. However the folding of the cable that catches on the body whenever the upper bezel strip was lifted and replaced eventually caused the ribbon cable to break. The cable isn't exactly cheap to replace either (which I reckon is an artificial price hike). As far as I'm aware the design for this has been improved on the M6500 and later. 

- Upgraded from 4GB of DDR3 ram to 12GB. Still room for another 4GB.

- Moved over to a full SSD.

- Upgraded the processor, but aiming to get it on the quadcore at some point. However it might be more practical to move over to an i7 on a M6600 (or later) at this stage for better/easier video editing.(and gaming!)

- The PSU - doesn't charge the battery anymore at all although it does supply mains power to the machine. Initially when the issue set in it would charge after half an hour of screwing around with the cable and being careful not to nudge the cable once I'd found the exact position it would charge in. 
- left heat-sink exhaust fan - it's become rattley and noisy. I haven't replaced it, but I was able to temporarily fix it by opening the machine and the actual fan module to oil it a little. I used machine oil from a pair of hair clippers, after which it seemed to work perfectly for 5 or so months to remain silent. However the issue seems to have recently returned.

- The firewire port - not so much of an issue or upgrade. but there were doubts about the reliability, speed and performance as regards to the Ricoh firewire port and Ricoh controller chipset. The main reason being that Texas Instruments were essentially the DAW industry standard for proven reliabilty as far as firewire controller chips were concerned for DAW use. However in 5 years of use (just over) I can say without doubt that the Ricoh firewire port and controller chip performed flawlessly. Very surprising considering that I better know Ricoh for high spec modular development hardware for digital camera's.

Things I would have liked to have seen on the precision machines...

Quick release SSD's/HHD's from the side of the machine? - The Precision range from the M6400 can accommodate 3 internal hard drives. I reckon a cool feature might be to have the ability to eject and insert SSD's/HHD's like a cartridge from the sides of the machine, on the security front it would mean that hard drive data was easily secured by the easy physical removal and porting of the BIOS encrypted SSD's/HHD's where porting the entire machine might not be practical. It would be a damn sight easier then having to physically open up the machine all the time. Special anti-static cases might be an idea too.
The potential to have the 3rd internal SSD/HHD cable of operating at full SATA controller speed rather than be gimped? - I'm not sure if this changed for later iterations, but on the M6400 the 3rd HDD/SSD bay (interchangeable with the CD-drive) was capped at half the speed. 

From what I'm aware much of what was learned from the M6400 was used to produce a vastly more refined machine for the M6500 release. No doubt the machine has gotten vastly better since then too. It was certainly the first of its kind to have a vastly more appealing external design. It's just a shame it's not getting thinner and lighter with the way laptop technology technology currently is now for ultra portables sporting i7's. I shouldn't think that it was impossible to potentially get the next generation of 17 inch precision machines as light and as thin as 17 inch Macbook pro's were just before the got canned. I've gotten so used to a 17" LED screen on the M6400 when using Photoshop or premier pro for editing, anything else just seems washed out and cramped whilst making it difficult to gauge colour depth.