I came from a 2001 4.4 E53 (died at 216k miles) and went to a 2014 50i. I also own a 2013 550i. I actually feel like the 50i is more sporty than my 550i. Might be the N63TU engine over the regular N63, but it's definitely sporty. The turbo lag is something you'll probably have to get used to, but once you tune your foot, it's fine. I really enjoy it. 21 mpg is probably optimistic. Bet on 19.5 to 20 mostly highway. The creature comfort improvements are huge, that alone makes it worth it. Find one loaded if you can, that helps a lot.

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2001 X5 4.4i Sport
2013 550i Xdrive M Sport
2000 528i E39 (Sold)
Unleashed Air Racing 279 mph Best Average Speed