The biggest problems:
Transmission - Terrible engagement from a stop. Kind of like driving an old car with a clutch that has a very skinny friction point. Hard to start smoothly. However, in the SLS, your hard to start smooothly issue is compounded by a 570 hp motor! So you wind up blasting off like an @$$hole from traffic lights and stop signs, without necessarily flooring it at all, it just does not engage and then it explodes. Big / huge problem. I did get better at fluttering the throttle over time, but it went from terrible to modestly manageable. A honda civic is more predictible off the line. It is a dual clutch, of the same type found in Ferraris, but MB has slowed it down for some reason and it does not engage gears quickly in manual. I found this really annoying. There are 4 teansmission programs, C, S, S+, and manual. The starting issues are less bad in S+, however that mode tries to redline the engine under even modest acceleration attempts. So I basically never found a transmission mode that I liked.
Width - the SLS is exceptionally wide. I have never seen a car other than a Rolls Royce so wide. It basically does not fit on narrower side streets.
Flashiness - This is a personal taste issue. I have never bought a Ferrari / Lambo or other exotica because of the attention factor. Myy old 997 C4S got no attention at all, one of the merits of Porsche's obviously. I had thought that a silver Mrec that resembled an SL would not be particularly noticeable. Big mistake - it got a ton of attention and much of it the not so nice kind. Other people will likely have a different experience, but I don't desire conversations about my vehicle choice or occupation or other things when I get a morning coffee.
Practicality - there isn't any. This is similarly a personal choice issue. But short of commuting to work mostly by highway (not my situation) and Sunday drives, it is not a serious option for much of anything.
To be clear- - the car has some significant merits. I thought the suspension setup and ability to place the car were excellent. You sit way the hell back - for a guy coming out of a 911 it is a particularly jarring change. But you do figure it out, and it is a distinctive ride. The power is explosive, and the noise cannot be topped. The cockpit is very cool. On the highway, in the mountains (only on a warm dry day), I would describe it in orgasmic terms.
I am going to try a new SL63 on for size - 95% of the capabilities, maybe more accelerative, does not have the transmission issue, might have more power at altitude anyway, and vastly more practical and non-attention grabbing. If that fails, I am definitely going back to a 911.