... can introduce major security flaws unless used *very* carefully
... reduces readability of your script
... is harder to debug
... is unnecessary 99% of the times it's used
... is the illegitimate child of Satan himself
... some people claim is slower than the alternatives
Other than that, eval is great
As for PHP 5.3 - it's nice n all, but in my attempts to use it, I've had more headaches than anything. I'm told 5.3.1 is what the cool kids use. I use whatever cPanel supports (5.2.11).