Whether or not you use it, it's all about understanding the tradeoffs you're making. It's worked out for us :)
If you want more from us about GAE, our lead developer Ben Kamens' blog is full of good stuff: http://bjk5.com
At WebFilings (https://webfilings.com/) we've been using since day 1 as well. It has it's warts but once you get some experience on the platform it can be pretty great.
Of course you guys are already at the millions-of-users use case, but KA is a (the?) great example of an app where GAE worked out from start to finish, which I find encouraging.