CSS people have been compensating for bugs for years without doing UA sniffing ;)
\r\n\r\nBut this is largely a red herring; my point is simply that UA sniffing to determine what features you'll let the user see is, and ought to remain, dead. If it's the most efficient way to solve a CSS bug, then that's another story, but the question here is things like whether you send your CSS to the browser at all, not whether you send the fix for a 2-pixel rendering bug.