I have been running Ubuntu as a desktop for a couple of years now. At work and at home. I am a gamer, and I have to say that for my purposes wine has been working superbly.
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At Home:
Last night, just for curiosity sake, I mounted my old windows partition (the one riddled with tons of game, ssshhh). I was able to run Spore, Elder Scrolls: Oblivion, WoW (duh), Thief III: Deadly Shadows, Vampire: The masquerade, Kingdom O' Magic, NWN : Windows Client (I have the linux client as well).
I was able to run all of these games with no custom wine modifications (except for KOM, which has to run in dosbox on both Linux and Windows because of its age). Played each for a level, or stage, or what have you. Some of them were slower than if they had been running straight from Windows, but given the development rate of wine, I wouldn't be surprised to see this lessened by the end of the year. I can get over 30 fps in each game, some more than others obviously. For instance, if I were to boot into my Windows XP installation and run nwn I get less FPS than if I booted into linux and run nwn natively. I wish I could run more tests, but I don't have any other games that have a native windows *and* linux client.
Oh, did I mention I have desktop cube (among other things) enabled while I played all of these?
I am very, very, happy with my 3 year old desktop running Ubuntu. With the improvements in the 2.6.29 kernel, I think things will get even better once I have configured and compiled this (currently using stock ubuntu kernel).
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At work:
I am using ubuntu on a full blown windows network. I'm talking exchange (no IMAP), SMB shares, etc (standard office environment). Evolution using OWA has a lot of problems, but it works. Changing your preferences a bit (Set Inline forwarding, Outlook style replies, etc) will make it emulate an Outlook install. It crashes pretty often, but thats probably because it has to interface with OWA rather than connecting directly with IMAP. Been running this setup for quite some time. I keep a file smb.credentials (rw-------)for easy rdesktop.
As always, Your Mileage May Vary.