A great article Chris Coyier wrote for .NET Magazine on some best practices and tips when using CSS3 on your website.
Yes, you should be using CSS3 today. Let’s skip right past that whole discussion and start talking about how we can write better CSS3. A lot of the new features of CSS3 are just plain more complicated. They have complicated syntaxes. We have to use strange “vendor prefixes” to get them to work in as many browsers as we can. We have to understand cross browser inconsistencies and stay on top of evolutions.
Another great point about performance issues with CSS3:
What I do need to tell you is that even if you do use good taste, many of these CSS3 features have page performance concerns. WebKit has a very cool keyframe animation feature (would you call that part of CSS3? sure, let’s.) If you animate just a few elements on the page, you’ll see some pretty decent CPU usage. Make it animate infinitely, do a couple different elements, and put other CSS3 features on those elements, and you’ll likely see some lag, some choppiness, maybe even some browser freezing.
Source .NET Magazine