I also have my 35is under extended warratty at 65000 miles at £52 per month and I had (another) cracked filter housing. Would have been £470 fix.I had the extended warranty on my 35iS, only used it once but it's worth it for the level of service alone. Came and picked up from my driveway, brand new 4GC courtesy car which I had for 2 weeks and all sorted with no input from me at all. The issue was only minor, but took a lot of investigation (hence the 2 weeks courtesy) which would cost a lot on its own.
On a sidenote, I had MHD Stage 1+ map on it at various times in it's life and BMW saw no signs of it. The chap did a very long scan on the car from his van and was moaning at the time about an M4 he'd just been to see which 'had a dodgy map on it', so it seems that MHD may be harder to notice. Anecdotal but there you go.
The 35is is full of expensive gear and has some common issues, especially the electronic shocks, turbo wastegates, fuel pump and gearbox oil leaks - all of them very expensive to fix - thousands.