Actually once non-run flats are on the SC430 the ride isn't rough at all.
In fact I found it far too soft.
Switched to a Tein CS coilover suspension with EDFC and Daizen sway bars, and it's STILL too soft! (19" Maya TS wheels BTW).
I've done a good bit of driving in the XLR and found the ride to be a bit more firm (a good thing for me), but cornering with the magnetic suspension is much much flatter.
I love the SC430, but the handling just isn't there and I'm considering either the SLK55, the XLR-V, or possibly the new BMW Z4 M Edition that's coming out.
SC430 = Steal of a deal, got it at $52k w/8k miles, soundsystem is incredible, handling far too soft, aftermarket remote control for the top is a superb feature that I can't get for the XLR. Back seat area is superb for cargo/groceries. Service is the best, they treat me like royalty and the Stevens Creek Lexus service facility in Santa Clara is just incredible. 36 lifts, floor is absolutely spotless, over 100 loaner Lexus vehicles, free coffee, espresso, lattes, cookies, pastries, internet access, big screen TV, nintendo for the kids, the list goes on and on. When I switch from Lexus I sure will miss the service!
SLK55 = Fair price at $70k loaded, Mercedes dealers are awful though, nav system is awful, great power, 0-60 in 4.3 will crush the XLR-V at a far lower price.
SL55 = Overpriced, far too heavy at 4200-4300lbs, nav system still sucks, looks great though.
XLR-V = $90-95k? You've got to be kidding me. Once the price drops as it did on the XLR it'll be worth the money, if it's in the $75k-$80k range sure, but $100k out the door for a GM? I don't think so. And no adaptive cruise control? Can that be added back on in any way? Can't believe they're dropping that. Love everything else about the XLR-V though, should be great power, great handling, great soundsystem, nav system, looks, interior and exterior. Certainly a great option when the price comes down.
BMW Z4 = I owned a Z4 with the 225hp 3.0 engine for a while, superb handling, nice ride, double the trunk space of my old Z3, but still not enough. Nav system is awful, they're absolutely DIRT cheap now. I paid about $53k for a fully loaded one when they first came out, the same car can be had for $25k-$30k now with 20k miles or so. Soundsystem is pretty poor though.
If they give the car a nice facelift, drop in the 321hp M3 engine, use the better SMG, keep the price not too high, the MZ4 might be a good option.
Another vehicle on the horizon? I'm hearing rumors that there will be a convertible version of the new Nissan/Infiniti Skyline GT-R. Possibly retractable hardtop, with a 450hp Twin Turbo VQ engine. Yum! Anyone driven the new Infiniti M45 Sport? Fantastic car. There's a new Lexus IS that will be out in a few months, and next year there are serious rumors of not only a convertible version, but they're thinking of droping the 4.6l V8 in there and plan to compete with BMW's M division on all fronts. Lexus IS460 with retractable hardtop? That might work too.
Either way, none of these cars are actually here yet except for the SLK55. Hmmm.