Why care about another PHP framework? especially one that's actually called Yet Another Framework?
Because Yaf isn't just a framework. It's a PHP extension (listed on PECL), so it's written in C -- so it's very, very fast.
Here's a benchmark (requests/sec):
That's Zend on the far left, then CodeIgniter, then Yii. And MicroMVC does way less, so it's not really comparable at all. As you can see, Yaf handles many times more requests per second than any of other the three frameworks benchmarked here.
I don't know much more about Yaf (whose manual seems to have been translated into English only recently). But from these benchmarks, at least, and from the fact that it's a C extension, it looks like -- well, if you're worried about performance, you might want to think hard about Yaf.
If anyone has used Yaf before, or is using it now, your insight would be awesome.