If (insert flavour of linux here) does everything a user wants, the question is why not use it?
Linux is a superior OS in performance and reliability--if your applications run on it. (It'll even run a better Windows network than Windows Server.) Ubuntu is very nice and I'd build a box with it for any client who didn't need/want Windows.
Personally, I'm a PC gamer and I use Netflix, both completely rule out Linux for me (no, I'm not fiddling with WINE to do 80% of what Windows already does without fiddling); if it weren't for DirectX and Silverlight, I would run Linux.
i5-2500K 4.2GHz | 8GB Mushkin DDR3-2133 | Gigabyte Z68XP-UD4, GTX580-882/2033 | Crucial m4 128GB SSD (64GB SRT cache) | WD 2TB 2002FAEX | Enermax Galaxy Evo 1250W | APC BR1500 UPS | Antec Twelve Hundred | When I was your age, I could outrun a centaur...until I took an arrow to the knee