Did use an iPhone 3GS plus MM last year for a transit in the front seat of an Extra 300. No problems obtaining a position fix, but the little heading indicator on the location bug remained resolutely at 90 degrees to the direction of travel.
Combined with the phone's inherent desire to turn the screen off to save power, the whole thing was a bit too fiddly to be of regular, practical use, and I haven't tried it since.
I do think Memory Map have missed a trick by not opening up the platform to third party extensions - they seem to spend most of the development budget on increasingly sophisticated licensing techniques, but the map viewers themselves haven't really changed in years. Had they invested in extension APIs, they would probably have seen a plethora of web-enabled active content available as overlays within the map viewer applications - NOTAMs, weather, airfield data from the aviation perspective, and the equivalent for the hiking or marine markets.
I did write an active MM route card template which would take a route defined in the map viewer, and use a web-service to get forecast winds aloft to generate a PLOG, plus a NOTAM narrow route brief, but never got round to releasing it for general consumption.