I don't have any personal experience with auto pfds yet, but we're in the market for them.
Practical Sailor has a review of 10 inflatable pfds without harnesses in the June issue:
Practical Sailor June 2008
Unfortunately, this link will only bring you to a teaser paragraph if you're not logged in as a subscriber.
It was a limited test of 10, Crewsaver, Float-tech, Mustang (hydrostatic), 2 Revere, 3 Viking, and 2 West Marine by Stearns. SOSpenders was not included. There was another review in April 2006, but I don't know which were included that time. There is a promise of a test of those with harnesses soon.
Ranked "best choice" were the Revere Comfortmax 61018 (manual) and 61019 (auto/manual) for comfort, price, ease of repacking and water-activated light. The Mustang ranked lower due to initial cost, rearming kit cost, difficult repacking, and two side wings didn't inflate automatically during the test. There was no mention of the required maintenance frequency.
The Comfortmax are rated type III and II, so they count toward the total lifejackets aboard even when not worn. (It seems silly to me that one brand would count and others wouldn't, because I can't see any functional difference between them, but CG regulations are CG regulations.... Good for them for getting the certification.)