i considered it in the past, and i might make the switch with all the internal crap going on with CI. but my biggest current project is CI-based and making the conversion would be...tough. plus, their docs suck.
They've rebuilt much of the base code on top of PHP 5 natively now instead of being built on PHP 4 with hacks to make it also work with 5.
Although it is still in development, it has been said that it is stable.