I don't really count myself part of any brand's mafiosi.. but I'd still like to come out to bat for the SLR, in a minor way.
Maybe I'm just being picky with your wording, but to me, 'compromised as a supercar' would really depend on your definition of supercar. Picking on you both, since there are only two posts here (and not flaming mind you)..
Performance wise, if that's the issue, it's not exactly slow, and neither is it too lardy. Sure it's not the best, but what's the cutoff for "supercar"? Or is it cos I is black? Er, I mean, is it cos it has an auto?
Looks wise, I agree that it looks aggressive and eye-catching and not particularly good, IMO. But, similar to what Voltaire was rumoured to have said, I disapprove of its looks, but I'll defend its right to look it - and to be called a supercar nevertheless (in another mangled French allusion - to eat its cake and have it too). Surely there have been uglier supercars, and will be. Besides, there seem to be some who truly think it looks good, though I don't know what possessed them to do so. Just kidding, sort of.

Not bad, but not good.
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Well, as for overrated, how would it please you guys if I said McLaren F1?

About how, for all its purity of purpose, its time has passed and it doesn't even look as great as it once did, being a good decade or so old. I'm sure that'd get all the other mafiosi in and we'd have a right royal romp in here. So maybe I won't, eh..
I'm ever for thinking that any car that people rave over is overrated. That's just me - whether it be Subaru WRX, Nissan Skyline GTR, BMW M5, McLaren F1 or whatever. I just get annoyed when people start idolising and stop thinking.
As for underrated, I might also say Audi.. otherwise, perhaps Lexus. I'm not saying its abilities are the same as other cars, in terms of dynamics, but there must be some reason why it's popular in America, and it's not because they're cheap (they're not). It could be the service, or whatever.. but the total package appeals. Maybe Americans have different priorities. Whichever, I also get sick of hearing Lexus bashing. Some people have their preferences, but IMO, 9/10 times, I'm just hearing the sound of closed minds.