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.
The car truly lives up to the reviews, with blistering performance and amazing handling all in its non-assuming package. My previous car was a 1998 M3 and this is certainly faster though a bigger beast for sure. The gas mileage is, well, what one would expect with a performance car this large.
I've driven 911's and Jag XK8 and this is by far the best of the lot. Doesn't have the stying of the jag but the incredible performance more than makes up for that. While the sound system is great I often turn it off to listen to the heart quickening sound of those 400 horses! There is no more fun than having granny waving from the back seat as I blow past the stunned guy in the vette next to me!