Let’s not be too naive: nothing’s free here. If you want to trade hosting costs to let Google keep in-depth stats on every visit and visitor to your website forever, as well as copies of everything you upload—then go for it.

But, before you do, here’s some highlights from the app engine terms of service:

Google can censor you
5.2. Google reserves the right (but shall have no obligation) to pre-screen, review, flag, filter, modify, refuse or remove any or all Content from the Service.

Google can lose your data
5.5. You agree that Google has no responsibility or liability for the deletion or failure to store any Content and other communications maintained or transmitted through use of the Service.

Google can use your brand for free advertising forever
8.2. You agree that Google, in its sole discretion, may use your trade names, trademarks, service marks, logos, domain names and other distinctive brand features in presentations, marketing materials, customer lists, financial reports and Web site listings (including links to your website) for the purpose of advertising or publicizing your use of the Service.

Google can drop your account whenever
10.2. … Google may terminate your use of the Service for any reason with ninety (90) days prior written notice.