thoughts on php5?

At work, we've been happily running php-4.3.6 for about a year, and
it's worked great for us.  (just for reference, we're running gentoo
linux and apache-2.0.52-r1)  Lately, we've been tossing around the
idea of upgrading to php5.

The main driving force behind this is our desire to use mnoGoSearch,
which, if I want to stay within Gentoo's portage system (which I do),
is only possible if we upgrade to php5.  Yes, I know, it's always
possible to download and compile php4 with mnoGoSearch support myself,
but I'd much rather stay within portage.  Which leads me to my
question:

What are your experiences with upgrading to php5?  Have you had any
major issues?  Any pitfalls/gotchas to avoid?

Thanks!
-Erik Anderson