Great MVC/OO framework. PHP4&5 compatible, good template engine, database independence, I18N support and still lightweight (beside bundled Pear libraries). It stands out of the crowd.
I made an e-commerce site using only the core (no CMS needed). It is very flexible and uses search engine friendly URLs. It is mature and suited to production use. Actively developed by lot of good coders.
The doc. is complete, the code is clear and reusable and the mailing list is helpful. Good choice! Try it.