<p>When would you use a bigtabe/simpledb database vs a Relational database?</p>

Size, redundancy and performance are all key words that pop up in my head.
It's fairly easy to create a little database that does its job but when you need to do significant scaling and redundancy, while keeping it performing at the speed of light, it's cheaper for lots of people to ship the job off to Amazon who've been doing it for a lot longer than most of us.