I picked up a Logitech G502 (Proteus Spectrum) at the weekend as Maplins had one out and were price matching everywhere (although they were cheapest anyway...!?)
Anyway, the mouse is wonderful but something I thought I wouldn't like but really do is the "non friction" mouse wheel. You can press a button and instead of having (rather loud) clicky scrolls, the wheel just spins freely on a smooth bearing. It's incredibly satisfying and makes rolling through large documents or spreadsheets a breeze - once you are used to the sensitivity.
I don't change my hardware that much so it's probably nothing new, but I was pleasantly surprised by the feature.