I think Cabo Marshall dives depend enormously on your guide. I had always had close up encounters with mantas, Galapagos sharks, sea lions, and the vast schools of salemas that envelope a diver enough for it to appear you entered a cave while from the outside of the school, the only evidence a diver was inside were the bubbles above the school. However, also went once when we swam hard against the current just to see a school of barracuda (which was usually present towards the safety stop on other dives). Rest of the dive produced little else and you really don't need to ever swim against a strong current. There are different locations at Cabo Marshall which is a relative large area, thus me saying the quality of Cabo Marshall has a lot to do with your dive guide.
Some are trying to get Cabo Marshall as an option one day. Sometimes, during high season, the visibility can be horrible at Cabo Marshall...I have received reports from guides that the viz was as low as 3 meters, but if I recall correctly, that was during the el nino/la nina event when the water is green and chunky everywhere. Since the water is much colder in la nina, more Orcas were present at many sites, so bad viz or no, had guide/client reports of watching Orcas prey on mantas during safety stop at Cabo Marshall. That would compensate for an otherwise lackluster dive, no?
Most 2012 itineraries should be out next week.