My workstation runs on XP (and I run two TWSs on it) and I don't plan on changing it in the near future. My laptop, however, came with Vista pre-installed and I keep it on my side during the day for web browsing, running a paper trading account to test strategies/have it display the watchlist on etc. While the sim account runs quite nicely I find that Vista itself is a bit unstable with driver conflicts (with other programs, not TWS) and whatnot and I wouldn't depend on my daytrading endeavor with it. Unless you're a volume trader I suppose it would be fine, but then again, I haven't even opened my live accounts on it yet.
In short, given the choice, stick with XP.