Windows 7 ≥ Kubuntu >>> OSX
Having recently reinstalled OSX 10.6 (along with Kubuntu 10.04 and Win 7) on my old macbook 1.1, OSX defiantly feels dated when compared to the others.
Both Kubuntu and Win 7 feel snappier, look fresher and are my preferd OS' when on the go.
Between Kubuntu and Win 7, both work very nice and I actually prefer Kubunto overall.
The only thing keeping it from my main pick is the lack of compatibility with Lightroom 3 and difficulty calibrating the monitor (ended up loading the profile created in Windows to fix this issue).
On a desktop I would also miss the lack of Steam and simple gaming compatibility with Kubuntu.
OSX could be fun to play with (just to know your way around another OS) but is generally outpaced by its competition.