I have never owed a Merc, driven a few, so can't really say much more than has been said. Merc or BMW? depends what you are looking for. In my opinion, these 2 co, even though competing for the same market, have different philosophies. Merc has always been for the gentlemen. A performance model for merc will be to take the normal car and stick in a big V8. Goes like hell in a straight line, but no feedback, no feeling, no soul. C55, E55, C63...huge pulling power.
BMW on the other hand, take more time in all round performance and handling. Just look at the E55, E Class with a big V8 and some mags that revs to 6000rpm, overtaking is a breeze, straight line speed amazing. Now compare that to what BMW did with the M5...as a performance model, the entire car is just amazing, the motor in phenominal (still my fav BMW).
If you want a fast, smooth vehicle, Merc is your car. Many would say that traffic, speed limits and all the other problems of modern day living make having a car that handles well pointless, so the Merc would be the best choice. But dont expect that when you leave work after having a crappy day, your Merc is going to put a smile on your face driving home. Might seem like something small, but for me, is worth it.
Just my personel opinion...