A proper review is difficult, for a number of reasons. Namely, there is a very vague understanding of the difference between this "Preview Release" and what we'll have when it launches with all official status and commercial potential. We can only hope that any big changes won't interfere with our existing applications. If that does happen then existing players might turn to AppDrop for help, although any mass migration is highly doubtful. In any case, what everyone is talking about is what App Engine is now, not what it might turn into at some unknown point. (As a side note, it would be really great if that point was less unknown!)
What is a review of App Engine really about? Their choice of included libraries, as well as the promotion of Django templates into such a defacto standard (already having Guido's blessing) is one thing to talk about. The differences between BigTable and traditional relational databases is a big topic of debate. We can discuss the development server and deployment method, as well as the control panel available to us, if we want to focus away from the actual programming for a moment. In the end, what we really need to care about is the thing Google set out to solve: experience.
They changed the experience in two meanings of the word. A less experienced developer can now make a successful launch. All developers can have a much better experience. The experience, both in history and present, is of the full cycle of development. App Engine isn't doing anything for writing code. That is all low-bar when you look at the tools they use that were already available and in wide-use, like Python and Django. The value added spice of App Engine is what you do when you aren't writing code.
People complain about the choice of language, if they aren't already Python lovers. Some people just don't like the choice of included template system. There are complaints about BigTable in App Engine and its problems compared to a "normal" database. These debates are all bunk. People are complaining about the very things that don't matter one bit. In the end, we might develop a hundred libraries for doing the same thing, because we all think we know the slightly better way to do it, but App Engine exposes our primary flaw: we're developers. We're great at solving problems, building solutions, and writing code. We run out of steam when it comes to doing something with it. Our problem solving desire is a largely academic one.
If a software developer solved world hunger, he would blog about it and move on to the next project.
App Engine would read that post and actually go out and implement the solution the developer forgot about. It does a great job at what it solves, and I love using it. What I'm doing with it and why not enough developers care about it are both stories for other posts.