I've used emacs a lot on Windows (2000), Linux and Mac OS X. I've never really noticed any problems with any of the implementations. I've used CVS for version control in Windows emacs. I haven't noticed any speed problems with the Win32 emacs. I've also used the cygwin emacs port (with X), which is quite good but doesn't copy & paste between normal windows programs I don't think (if memory serves) and isn't as fast (the X implementation is quite slow). If I have to use a computer for any length of time (regardless of OS) emacs is one of the first things I install on it. Can't get far without a real editor.
For MacOS X I'm using the X11 emacs from fink, though I had to recompile it to get X11 support (not sure if that's still the case). Since then I've also installed the Carbon emacs port on other machines, which seems pretty good, though I don't like the default font.
(on a side note, now that I'm used to emacs keybindings (ctrl-a, ctrl-f etc) I really appreciate the fact that they work in many MacOS X programs (anything Cocoa). I find it frustrating when I use programs that don't have these bindings now)