No personal experience of an E61, but last year I bought a manual E91 (325i Touring) and found it felt a bit underwhelming in the performance department. To the extent that yesterday I collected an E90 (330i Saloon) with the rare option of split-fold rear seats - what a difference another 40 bhp makes!

(It was exactly 100 miles away too). :lol:
These 2 were both early models so have the N52 engine, which as far as I know are only let down by the lack of a dip-stick and the cost of electric water pumps.
Petrol E61s from 2003 to 2005 had the M54 engine, then from 2005 to 2007 they had the more powerful (and lighter) N52. After 2007 they got the N53 engine, which I specifically didn't want after reading many tales of woe regarding failed injectors, high pressure fuel pumps and coking up - all to save a few quid on road tax and slightly better mpg. :roll:
Plus if E61s followed the same path as E91s the N52 engines come with hydraulic PAS which is so much better than the electric PAS on my Z4C.

I'm pretty sure N53 engined E61s will have EPAS - for chasing low C02 emissions.
I don't know much about the diesels, but did run the idea of an M57 engine past my BMW Indy a year ago and he said an N57 would be a better idea if I wanted an easy life!
Then there are the 535i (which I think will sadly cost over £500 p.a. for road tax) and the 535d (which is auto. only) but that's all I know about those. :roll:
Hopefully someone more knowledgeable will be able to help!