I use app engine for several projects, a couple of which have decent load. My biggest one is networkedblogs.com, which I started on php/mysql and recently migrated big parts of it's back-end to the app engine. My experience with it has been good, and I always build new projects on it, unless there is a good reason that prevents it. It has it's own pros and cons, but overall it's a good choice:<p>- It's easy to start with and fun to use. You shouldn't need more than one day to learn it.<p>- If you like to focus on coding and not worry about managing the server then you'll love it. But if you're a control freak and like to have access to low-level settings then you'll not.<p>- If you have prior experience with relational databases such as mysql, then you'll need an adjusting period to forget much of what you learned about databases and start thinking in new and different ways. Some things that are really easy in mysql, such as getting the number of records in a table or joining tables, become hard on app engine and require writing code.<p>- When it goes down, you'll feel helpless that you can't do anything to bring your site back up, but you'll also feel lucky that a Google employee is carrying that beeper and have to wake up and fix it.<p>- It scales, but it's not super fast. Due to its distributed nature, individual requests rarely finish in less than 50ms and usually take 100 to 150ms (that' my experience, yours will likely differ). But it can run a huge amount of requests at the same time.<p>- It solves the scaling problem, but it's not magic. You still need to optimize the heavily used parts of your code for scaling. But at least you do it with code, rather than having to fiddle with server and cache settings.<p>- It's a paradigm shift that will eventually eliminate most of database and system admin jobs if it catches it (I think it will).<p>- It's very different from Amazon's EC2. Amazon gives you servers that you have to manage, Google manages everything for you. With ec2 you pay per server hour, with App Engine you pay for resources you use: CPU Cycles, storage, bandwidth, ...etc.<p>- It's free to start with, but if you have heavy load it's a bit more expensive than hosting dedicated servers. I think it's worth it, though, because you'll be saving a lot of your own time.<p>- It's good for the majority of projects, but if your project has special not-too-common requirements, then it might not. Check first.<p>- It only supports python and Java right now.<p>If you're in Silicon Valley and if you need more in-depth information, you can find me at the next App Engine meetup at Google on the 6th.