I bought it because there really is nothing else in its class. If you want a very fast and very comfortable big sedan with a 6 speed, this is your only choice. I owned an M3 before, and the M5 is much more comfortable/luxurious. The M5 does give up quite a bit of agility to the M3, but it makes up for that with the quiet and comfortable ride. Have a very thorough pre-purchase inspection done as there are some very expensive parts on this car. Look for a well-maintained car that was owned by an enthusiast. Reliability is very good for a car in this class, but you will be in for a surprise if you are not used to BMW maintenance/repair costs. They need all cooling parts replaced @ 5 years.
Luxury /comfort (nice sound deadening, thick glass, comfortable seats, not rock-hard suspension, etc.), 6 speed manual, nimble for a car of this size, 19+ mpg in mixed driving, looks 10X better than the 540i.
Reliability is pretty much middle-of-the-road (about like a GM, Ford, etc.) when compared to all other cars, but reliability is great when compared to other performance sedans (Maserati, Audi S6, Jaguar R, Mercedes AMG). Would be great if it were as reliable as a Honda, but then it would be a Honda.
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