My girlfriend has an Iphone 4 which I set up for her.
Then (last Dec) there were two versions of MM for sale in the Church of Jesus Jobs online shop: one for about £10 and one for about £25.
The £10 one didn't do anything as it came but you could download maps into it.
I bought the £25 one, which has a feature whereby the Iphone becomes a web server (or something like that; I've forgotten) and you can transfer the standard MM QCT format maps to the Iphone, from a PC.
That £25 app also automatically downloads street maps (the O/S 1:25k ones, apparently) around your present position. I don't know how long this works for - MM never reply to any emails. It is similar to the free Google Maps app on the Iphone, though arguably more readable.
I transferred a load of QCT maps I had to it, and they work fine.
The MM app is a bit clunky but does work adequately. But you don't buy MM for the program, you buy it for the maps
The CAA charts are now sold in the encrypted QC3 format which is locked to the device so it cannot be copied. I have no experience of this format and would not touch any such locked products with a bargepole
I used to buy the 1:500k QCT maps each year (they were about £20 each i.e. £60/year for all of UK) but stopped when they went to the secure format.
Luckily there are various (evidently scanned from print) QCT CAA charts floating around on the P2P scene, including one for the whole UK.
The GPS in the Iphone is excellent on the ground. I have not tried it airborne but someone I know with an Ipad reports that it loses the signal easily. Unfortunately I don't think you can run an Iphone with an external (bluetooth) GPS without jailbreaking it.