Pros: Great it you want a speedier network connection on older laptops or some of the newer netbooks with only 10/100 network card built in. Especially useful if you have a NAS since 100 mbit is just a bit on the slow side.
Don't need the driver CD to install. But with XP you need to allow it to look for drivers on the internet. With Vista I don't know but I'd be really surprised of it doesn't work.

Cons: Seems to get a bit warm when running gigabit (which is probably why you've bought in the first place). However nothing I'd be worried about safety wise but heat = energy = might give noticeable shorter runtime on battery.

Other Thoughts: Think twice before using this for an older laptop. It might not be faster than the built 100 mbit network card since USB puts a strain on the CPU at higher speeds. So that depends on how fast your older laptop is.

Bottomline: Buy it of you want a speedier connection on those popular netbook or perhaps for an older laptop with fast CPU.