Speaking theoretically, Dynamo is some really clever tech built by some very smart people -- it's clear that Amazon are using something very similar internally, so it must work in practice. Beyond that I've no direct experience with it or its performance profile.

If I had a very large, rapidly-growing key-value application and a shortage of experienced ops engineers that made maintaining my own solution impractical (e.g. a cassandra cluster) I would look hard at dynamo.

However, as a matter of principle I am very suspicious of the lock-in that comes with proprietary solutions, no matter how clever. We try not to buy cloud services that only have one vendor.