We had a 55 plate 320D Touring for 4 years and whilst not being overly impressed with it, I certainly wouldn't knock it.
We did have two very expensive bills which were the satnav and the turbo, but, it was always serviced at Sytner BMW, and I've come to think (over the last year) that they are rip off merchants to say the least! With the satnav, apparently they just take it out and send it away to Germany for analysis, simply replacing the whole unit. Whether this was necessary or not is moot, but, the fault appeared to be a bad connection/dry joint. A new satnav did sort it out. What annoyed me was the fact that they sent my (almost new) satnav disc away with it, and it took Sytner almost a year before they finally caved into repeated requests for its return, and gave me a new one.
The turbo blew, or so I thought, and when I got it home I discovered the inlet pipe had come loose due to becoming worn at the clamp. I shoved the hose back on and the car drove fine, but I took it in to get it checked as I didn't know whether the hose needed replacing or simply properly re-attaching. Needless to say, Sytner told me the centre bearing on the turbo was noisy and the entire turbo needed replacing. Naturally.
Those two expensive bills apart, and mine had every toy available, I found it to be well built, good looking in the Touring version, and pleasant to be in, although I would not recommend the automatic as I feel it seriously detracted from the driving experience. I averaged around 30mpg in very much short run journeys/daily commute, which I didn't think was outstanding for a modern 2l diesel.
Would I buy another E91? Yes, but, in a manual if I can find one, and I would probably buy a petrol for my current usage, and, if over 60K miles, I'd get it serviced by an independent, not the dealership.