What's the best way to compare frameworks? Probably not just by feature-set -- unless you're already quite sure what features you need, and, coincidentally, only one framework has them all.
Besides, manually trundling to every framework's website, and finding each framework's feature-set (not all of which are presented in the same format), takes some time; and you're never quite sure whether you missed something important to you, just because the framework creators didn't think it important enough to highlight outside the formal documentation.
Ideally, perhaps, the Great Guru of PHP would write a nuanced, lucid, and battle-tested comparison article -- every time a framework updates -- which could then be applied to your needs by way of sufficiently robust semantics, like an idealized expert system for comparative ratings. So many frameworks update so quickly, however, that this isn't going to happen any time soon; and the semantic web isn't there yet anyway.
Meanwhile, though, there's this wiki-ish comparison of PHP frameworks from socialcompare.com -- presently incomplete, but already containing quite a few frameworks and interesting details of each. You can add comparison criteria (rows; folksonomically) or update existing criteria -- which currently include, among other details, ORM specifics, cache storage, template system, and testing libraries.
One of the neatest criteria already present: the 'notable project using it' row. Now you know that Yahoo Answers is powered by Symfony (impressive!), for example.
The comparison site already contains a good chunk of info, but looks like it could become a voluminous and ever-updated repository for user-generated evaluation, if the user-base grows sufficiently. Definitely worth checking out, and maybe contributing some insights of your own.