As far as casual 2D desktop usage is concerned, ATI is probably fine. The problems, for me, come when needing hardware OpenGL drivers. ATI has been working on it and seems to possibly be improving, but in years past, ATI's support for OpenGL has been a crap shoot at best. First, was whether OpenGL even worked on whatever ATI card you put in. Next, was what bugs to be expecting. Basic things like gl_FragCoord (in fragment shaders) were completely broken...for a long time. And there were several other bugs a developer had to deal with. As an OpenGL developer, you pretty much had the following expectations:
Nvidia driver: OpenGL worked as spec'd. Very few if any bugs.
ATI driver: OpenGL was a crap shoot. Sometimes worked well enough. Sometimes had a ton of bugs that needed working around.
Intel driver: Revert to _software_ OpenGL rendering because Intel's drivers were so completely useless.
If a new extension comes out, or a new version of OpenGL comes out:
Nvidia: expect a new driver within weeks to support it.
ATI: expect support somewhere 6 months to a year later.
Intel: OpenGL? what's that?
ATI seems to be working to improve their situation, but they're still not quite to the level of OpenGL support that NVidia has provided.