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.