Well, I have had the SL 55 for just about two weeks, and I have had two E39 M5s in the past two years. I figured, I would give a pretty detailed analysis of the SL 55 as it compares to the M5 (even though they are two totally different beasts
Engine, Power, Brakes, Handling, Transmission:
1) The exhaust note in the SL is anything but "subtle". This car is louder than the kellners exhaust for the M5. The SL has loads of torque available throughout the rev band, thanks to the 5.5 liter engine and the supercharger, noticeably more than the M5 according to the buttometer.
2)The mechanical feeling of the brakes are very mushy. The car does not have the same "bite" as the M5 or the M3. This may be due to the Brake by wire technology, which, I feel, removes the driver from the car as well as the road. However, the car does brake remarkably fast. It just doesn't feel razor sharp. The feeling of the brake pedal is similar to an american rental(i.e. Pontiac, Chevrolet, Olds)
To sum it up: The brake "pedal" feels dull and mushy, but it stops the car remarkable fast
3)The handling of the car is not bad. It has gotten alot of crap from people, but I think it handles as good as the stock M5. According to Nigel M5, the SL 55 handles as well as his Dinan M5. The problem, once again, lies with the techno gadgets (Active Body Control in this case) which disconnects the driver from the car and the road. Even though you are taking the turn at 80 mph, the centripedal force does not quite translate onto your body the way it would in the M5.
The ESP (stability control) in the SL is alot more driver oriented and not as intrusive as the DSC in the M5, which is a huge plus.
The steering feel in the car is, plain and simple, numb. The SL 55 steering is far inferior to that of the M5. when turning, the steering on the SL does not give you the resistance that the M5 does. It wants to wobble left and right, makng the steering feel very light. Hands down, the M5 feels better driving in the twisties
4) Auto transmissions suck in general. But if I had to drive one, I would pick the MB slush box. They have done a great job of giving the driver greater control in the shift patterns. There are three modes (Winter, Sport, and Manual). Winter and Sport start the car in second gear (to minimize whiplash). Manual mode allows the driver to control the shifting via the steering wheel. However, there is a noticeable lag time between pressing the button and the changing of the gear. In manual mode, The buttons on the steering wheel allow upshift and downshift. If I want to downshift from 4th to 2nd gear, I would have to clearly and slowly press the button twice, otherwise the car will not read it properly and only downshift once. Clearly, not acceptable during sporty driving as everything happens very quickly. The MB Auto tranny is far inferior to the BMW SMG. The Achilles heal of this car, is the sluggish transmission. If I floor the pedal while cruising in the straights, and the transmission does not kickdown to the best possible gear, I may conceivably lose a close race. Also, I think the M5 has a stronger second gear than the SL. The SL has an insanely strong first gear, a respectable, but not jaw dropping second gear and into the first 2000 rpm in third gear, and then the speedo tach just climbs like its gone mad. SO, between 30mph and 85mph, the M5 may be a bit stronger.
Inteior, fit/finish, Nav, etc.
1) The interior finish in the SL is topnotch. Claims made by journalists in the declining quality of workmanship in MB cars cannot be corroborated here. There is beautiful stitching seen throughout the console and seat area as well as an abundance of alcantara (roofliner, above the speedo, on the back of the shiftknob). The Burl walnut is not as dark and nice as the M5. The MB burl wood is a bit redder like the BMW vavona redwood.
The Navigation in the MB sucks! Even though it is much faster than the BMW (similar to DVD nav in BMW), the feautires are not laid out as nicely as in the BMW NAV. Controls are totally unintuitive. The cupholders are sturdy! There is no way a cup will fall out of the holder when accelarating (which was a common problem with my M5 cupholder). However, the position of the cupholders in the MB are terrible, as it totally blocks out the NAV screen. This is another testament to the fact that MB engineers go for the most effective solutions in their design parameters, not caring if it restricts the driver in anyway. BMW chose to put whimpy cupholders because anything different would impede the driver's access.
PDC sensors in the MB have an audible and a visual warning system. Also, the sensors in the front and back can audibly signal at the same time, whereas the BMW sensors would interchange between the front and back when getting to the solid tone.
The sound system (Bose) pretty much sucks. I am no audiophile, but I would guess the DSP in the M5 is much better, especially with the ability to adjust the equalizer.
The steering wheel in the M5 is far more comfortable than the SL. The buttons on the M5 steering wheel are far simpler than the MB, which allows the driver to pay attention to driving more effectively. In the MB, if I wanted to change the CD track from the steering wheel, I would have to scroll through the menu until I reached "CD" and then change it. If I wanted to see the outside temperature, I would have to scroll through till I got to the temp. reading. Again, Mercedes does not factor the art of driving spiritedly nearly as well as BMW.
Seats are extremely comfortable, more so than the M5. The active massager is a plus, as is the ventilated seats.
There is a "sport" button in the SL. It is called "ABC sport". All it does is it stiffens the suspension for a firmer ride. It does not stiffen the steering wheel, nor does it open the valve butterfly for a dramatic increase in engine response, as is the case with BMW
The NAV CD in the SL loads in the cabin above the nav screen, and it also serves as a 7th slot for audio CDs, a big plus
I have only tested the SL against a couple of my friends' E46 m3. From 0-80 I had about 1 car length in a straight path, and pulling away hard! from 60 mph (3-2-1 signal and go) the SL showed its dominance right away as it slowly gained until about 75 and then it ran away. I have not had a chance to do a couple of runs against M5s, but I am sure NigelM5 and Rocket will easily oblige
TO sum things up:
1) exhaust note: big edge to SL
2) Engine Performance: SL
3) handling: even
4) brakes: even
5) steering "feel" big edge to M5
6) Quality of fit/finish: slight edge to SL
7) transmission: big edge to the M5 Manual tranny
8) Most luxurious/comfortable ride: SL
9) Sportier ride (i.e. more fun around the tracks): defintitely M5
Thank you guys for reading and I apologize for the extremely long write-up.