Streak co-founder here - yes, we're very surprised more startups don't use AppEngine. We think its mostly because it isn't noted as sexy.

We've been on AppEngine for under a year and its been great. Its been much more stable since the HRD (high replication datastore) launch and the SLA was made available. We rarely think about how we'll scale some feature or our app - we've moved from megabytes of data to terabyte scale without many changes at all.