Google itself is using App Engine on both internal and external projects as far as I know.
The websites below has tens of thousands of registered members, there are also similar websites for some other European countries (Italy, Spain, Germany etc)
So yeah, App Engine seems to be suitable for big projects if you play by their rules. Only problem seems to be backing up data, as their App Engine's bulkloader tool has rate limitation, so it takes quite a while to download/upload all data.
The websites below has tens of thousands of registered members, there are also similar websites for some other European countries (Italy, Spain, Germany etc)
Getting British Businesses Online: http://www.gbbo.co.uk/
Getting Australian Businesses Online: http://www.gettingbusinessonline.com.au/
So yeah, App Engine seems to be suitable for big projects if you play by their rules. Only problem seems to be backing up data, as their App Engine's bulkloader tool has rate limitation, so it takes quite a while to download/upload all data.