As someone who has grappled in the past and is grappling again with the issues of pricing a database-as-a-service though, this is very much a non-trivial issue. If you have nice scaling characteristics and you want to charge the minimal price, your price isn't going to be predictable. Essentially, what Amazon have done here is to set a cap and then charge you the cap irrespective of actual usage, which seems to be the model that's winning on the cloud.
SimpleDB's Machine hours are basically the same units, but without the throttle.
So, from a technical and value viewpoint, it's a huge step backwards (pay for capacity rather than for usage), but I'm learning that psychology is perhaps just as important here.
It seems like what you really want is a throttle with per-query charging, to cap your bill. Probably you'd much prefer not to be forced to pay your cap every month, but I don't think that's being offered.
(Edit: Downvotes? Am I wrong here? If so, please contribute to the discussion and tell me why!)
The big plus for Capacity Units is that Amazon actually provides a deterministic model for figuring out what you'll be charged for a given query.
This is a big step forward in transparency, although I would suggest that SimpleDB's pricing shouldn't have been obscured in the first place.