I got the Z680's about a little over a year ago and I love them. Not only are they really good speakers for you computer you can also hook them up to many different things. For example I have my PS2 connected to them through optical wire. They sound great! and from what I have been seeing lately they have really come down in price.
Oh yeah the logitech people rock, so if you have anything that goes wrong with them they will gladly send you a new set. I don't know how the other companies are about that but that might be one thing to just think about.