I've been using the Zend Framework since ver. 0.2 and I can really credit a lot of my growth as a developer to it. Obviously not perfect, it's still the best written PHP framework on the market.
to Thorpe and others who experienced performance issues - I can't comment on your specific situation, but from my experience the framework was never the bottleneck. One of our ZF projects scaled from one dedicated server that was squeezed to the max to now 28 servers and growing. We encountered database bottleneck, apache bottlenecks, memory bottlenecks, network issues, but the framework was never the problem. If anything, the high maintainability of code written with ZF has allowed us to quickly evaluate and optimize bottlenecks easily.
I'd really recommend the ZF to anyone who wants to get started with a framework - just a word of caution, there is some learning curve involved. Just brace yourself and you'd reap the rewards in the long run.