Amazon DynamoDB is a new hosted NoSQL service. It's replicated across at least 3 data centers, scales automatically, and has low latency. Really nice.
But the query capabilities are very limited (comparable to simple key-value stores without secondary indexes). That means extra work on your side to emulate secondary indexes - which you'd get for free with other NoSQL solutions. I wish they'd at least provide manually specified secondary indexes (similar to App Engine), so they don't need to index everything, but still make development very easy.
Also, I'm not sure if the costs are acceptable because the pricing is difficult to compare to other services and hosting on your own servers. For example, is $1/GB acceptable? App Engine's datastore is also replicated across at least 3 data centers, so how does DynamoDB's $1/GB compare to App Engine's $0.24/GB? Is the latency so much lower that it causes the 4x higher cost? Does App Engine have a higher overhead, thus causing much higher costs than the $0.24/GB might suggest?
At least, DynamoDB looks like a more attractive alternative to SimpleDB. Complexity-wise it makes scaling much easier, but queries/indexes much more difficult.
SimpleDB vs DynamoDB vs host some other NoSQL DB yourself? Pick the lesser evil. However, if we had DynamoDB with secondary indexes that would be really really interesting.