I'm not about to get into the in and outs of why you should or shouldn't use it although I will say that there are a lot of commercial entities that use bit torrent as a legitimate means of distributing their software. 

Regardless of which torrent client you use there are some major drawbacks to having a torrent client installed directly to a computer that you might use all the time with all your personal files stored on it. 

For most people once they install their bit torrent software of choice they won't notice much impact on system performance in the initial stages, unless that is they're using a machine with a single core processor with less than 2GB of RAM, in which case the hit in performance is usually very noticeable straight out of the blocks. However even if you're using a multi core system (of 2 or more) a very noticeable reduction in system performance will quickly set in after a couple of uses of your bit torrent software. This is especially so the case when you're connected to the internet and even if you don't have your bit torrent software loaded up actively downloading stuff.

Depending on which bit torrent client you use, it can open anything between 2 to 6 network ports for dedicated bit torrent use, and regardless of which bit torrent client you use once its installed it will always run 1 to 2 background processes regardless of whether you have the bit torrent client actively loaded up for downloading/distributing stuff or not. For most people as long as the bit torrent software is installed it will always be active whether you actively start the app or not. This also contributes to potentially serious system slow down whilst you are connected to the internet even if you're personally not using the application at all. 

However in Vista, windows 7, and Windows 8 its possible to manually close tight the ports that the bit torrent software uses whilst you are not actively using it via their built in firewalls (which XP doesn't have), and if you do close the bit torrent network ports you will notice your machine significantly speeding up. If you actually uninstall the bit torrent software too you'll notice your computer speed up even more. 

The point to this? Bit torrent does more then allow you to bit torrent stuff, it allows for additional discrete access to your entire computer over the internet.

 
Recommendations

Don't run bit torrent directly from a machine with all of your personal stuff on it, unless you do actually want to allow additional access to it, in which case go ahead. If you have a separate machine that you don't use it might be an idea to set that up as a dedicate bit torrent client instead if you're heavy into bit torrent.

… or…

If you want to run it from the same machine (for many valid reasons of power saving, cutting system redundancy, space saving, better efficient use of hardware resources that would otherwise never-ever get used just mention but a tiny few...) but still want to retain system performance when not using the bit torrent client and a higher level of privacy, I'd recommend setting up a virtual machine and running your bit torrent software inside a virtual machine. That way all additional access through the bit torrent software is limited to the virtual machine only. When you're done downloading via bit torrent you can simply move the stuff you've downloaded in the VM to the machine or the main host OS that you want to use the downloaded content on, then you can just simply close the virtual machine down with out it leaving constantly running background processes on the go or allowing for additional discrete access to cause your computer to seriously slow down. When you want to bit torrent again just load up the virtual machine which will keep everything separate from all your stuff.   

 
Where to get the stuff to do it for free on the virtualization front....
  

Setting up a dedicated bit torrent client is simple enough since you just need to use a single machine for bit torrenting only. However If you're looking to go the virtual machine route you can use free virtualization software like Virtual box which doesn't require registration or VMware player which does requires registration before you can actually download it for free. You'll also need a copy of Windows that you can activate whether that be XP, Vista, 7 or 8 for setting up your VM, but its also perfectly possible to do it all completely free via a distro of Linux too. In which case I reckon the latest version of Ubuntu might be the easiest to use for most people.