I have 2, I have a Compaq 4 port server type rack mount type monster. It works well but will only run 1280*1024@60hz (OW!) and wont work with wheel mice.
I also have an IO-Gear 2 port with cables built in that works great.
I have some suggestions on what to look for:
Check max resolution/refresh, leave yourself some headroom.
Make sure that it will support all the mouse functions you want.
Verify cables. Some come with them and some don't.
Make sure that you can switch via the keyboard so you dont have to flip a switch or something.
http://www.iogear.com/products/product.php?Item=GCS62
Thats the one I have and for 2 PCs it works great. I paid about $30 at Frys and got a $10 MIR (that I actually got)
I like Omardeth's suggestion:
Includes four KVM cables (1.8 meters/ 6 feet)
Support Microsoft Intellimouse, Logitech Net Mouse, Optical Mouse
up to 1920 x 1440 with 200Mhz bandwidth
PC selection via Push Button and Hot-Keys
Once you have one you will never go back. No more alt-tab while playing games. Just wack a key and use the other PC.
edit- to add praise for Ormadeths find