I'd disagree with them being expensive to maintain. Parts are reasonable, it's just the labour cost of garages. But all decent garages charge £80/hr+ for labour, doesn't matter if they are working on a £20k Golf or a £40k 3 series.
A belt change is a bit of a pain because you need to remove the fan and the fan requires you to remove the undertray. The belt itself is only £20 but it'll take 1.5hrs to change just because it's hard to reach but that's pretty normal on most cars as it's not something you change often.
Overall the 35i is pretty easy to work on even with the big engine, you can replace the coils and spark plugs in about 20minutes and oil changes are simple. Even bulbs are easy to get to which is something that requires a bumper removal on some more mainstream cars.