I've had these PHP 5.3.3 packages installed for quite a while: http:// rpms.famillecollet.com/el5.x86_64/
No problems at all but I went back to 5.2.10 from CentOS Testing because for what I needed it for (phpVirtualBox) 5.2 was sufficient and I did feel a bit better knowing where the 5.2.10 packages came from.
Can't tell you anything about reliability of famillecollet though, I just googled something about updating PHP on CentOS 5 and their site was one of the first ones to pop up.
Best, Martin