I've used Zend Studio IDE and NuSphere (actually, I own licenses to both) and have to say, even though NuSphere is not written in Java (like Zend) it's slower than expected. I like NuSphere; however, it's not as robust as Zend seems to be.
The parsing it uses to check syntax takes longer than Zends. So even though you can put an error in on purpose, it still takes like 5 to 10 seconds to find it, not Zend. Zend is almost instantaneous (on my computer at least).
PhpED has an issue with auto-closing brackets and quotes. They've 1/2 fixed it, and I'm not sure they're going to finish fixing it. Right now, it works for quotes (it will skip over the quote if you press the key to close it) but for braces, it just keeps on adding on.
If you're looking for a good alternative to Zend, this is it. Version 5 is nice, but I'd rather use Zend as it seems more mature than NuSphere. But I use each equally. Some things I do in NuSphere PhpED, others I do in Zend, and even others I do in Komodo 