Recording can be a little PC intensive, especially two shows at once - so using your computer while those shows are being recorded is probably going to be slow - you won’t be playing any graphically intensive games while recording your shows.
Now - final verdict - do you need one? Well, it depends. If you already have a DVR and you are happy with it, probably not. If you want to be able to save your shows longer than you have space to do so on your DVR, or if you’d like to be able to carry your shows around in your pocket - then it could be a benefit, and the Hauppauge WinTV-DCR-2650 is one of the least expensive and yet capable and easy-to-use options out there for these CableCARD USB devices. If you have a DVR and want to be able to record even more shows than you can now - pick one of these up.
If you are a Mac user - unless you are ready and willing to BootCamp Windows 7 - I am sorry you need to find another solution. If you have Windows XP or Vista (WHY DO YOU STILL HAVE VISTA?!?!?) you would need to upgrade before you can use this.
A note - CableCARDs are not generally well advertised on cable provider’s websites - basically because they don’t want to provide them - they want to lock you into their system, with their hardware. They are, however, required by law, I believe, to provide them. So even if you don’t see CableCARDs listed anywhere, call and ask, or email or web chat, whichever you prefer. They have them, they aren’t free, but they are offered.