Why "not" Google App Engine?

I've heard about GAE and even though it doesn't support PHP, people got around this limitation by using Quercus, I guess (I haven't tried it, but presumably it works). But what factors should be considered when assessing if this is the right fit for me? For example, what led you to fail with GAE? Or, if you went with him, what doesn't please you?

thank

Edit:

Thanks to those who saw the meaning in the question and defended it. I saw questions here that were much less programming-related than mine, and that were left open. From the point of view that this is cheating, it is in no way. This other question was specifically about PHP support; my question is why not GAE at all (php was sidebar). I doubt the guy who yelled "cheat" even read both questions to see how mine is obviously so different. You can vote for it again if you still see meaning in it. Some good answers are already coming, it's too bad that 5 of you killed the Perfect Good Thing. I voted to re-open, 4 more people could jump if interested.