As SARS said the roof mechanism is the same on the E93 as the F33 so most issues would have been sorted.
Had the dreaded water in the rear panel, it can be done at home with a tiny bit of spannering, (Take a look You tube) or get the garage you buy it from to sort it, if buying privately my garage did mine for £60 and its a permeant fix, just get it done from day one as it can also affect the electric rear quarter window motors which fail and can stop the rood lowering as well as the amp.
Touch wood, my roof has been faultless and the transformer thing is a gimmick IMHO, I've used mine once as I never leave the car anywhere with the roof down, not even at home.
Just make sure you get one with the air collar / heated seats / wind deflector as the car can be driven in colder weather with the roof down.
The issue is, no one ever comes onto a forum to say my roof has been great - its always, i have had an issue how do I fix it, I bet there are plenty of cars out there with no issues.