Any sort of emultion software is never going to be as good as the real thing
This is probably tired and redundant, but just in case: Wine stands for 'Wine Is Not an Emulator.' It's actually re-implimentation, so there's no overhead of an emulation layer. Whether this is better or worse than the original set of Win32 libraries will depend on the quality of each respective body of code.
I use WineX (transgaming's fork of the Wine project) to run many Windows games.
Until a year ago, WineX was noticably slower than Windows for games. But these days, people (myself included) report framerates that are comparable with, and sometimes faster than, running the game under windows.
However, the caveat is that the WineX project is not yet complete. Some games do not install (due to installation or copy-protection code requiring some functionality from Windows which Wine has not yet implemented.) Another common failure is for video playback, particularly if the game used some proprietary format of video.
But if you have any interest, the project is entirely usable - I use it on a daily basis with my favourites like GTA-VC and Thief. Many newer games are listed on transgamings website as working fully, too.