At first I figured I didn't want the R-Spec because it's not a true sport sedan. It doesn't have sport seats, adjustable ride, shift paddles, sport exhaust, etc. But I liked its stance with the larger wheels, and I'm a sucker for horsepower and 429 is a LOT of horsepower.

What clinched it for me was that the price premium over the 4.6 was so small. I ended up thinking, "if you get this car and own it for many years, you'll kick yourself that you didn't get the R-Spec."
My main concern was the ride. I knew that the Genesis in past years had some knocks against in reviews it for ride quality. While I read that the ride had been improved for '12, I also read early R-Spec previews where a couple of reviewers didn't like the R-Spec's ride, saying it was a bit stiff/jittery. I ended up driving it and being extremely happy with the ride. It's important to drive the R-Spec over roads you know well. It's entirely subjective. Don't let other people tell you that the ride is fine or that it's unacceptable. Give it a good test over roads you're familiar with in your G35 and decide for yourself.
So was it worth it? Well, every time I get into the gas in my R-Spec I smile. It builds speed so effortlessly it's just wonderful. I love always having ample passing power. And I've discovered a special kind of joy that the R-Spec is a sleeper. It doesn't announce itself with hood scoops, spoilers or coffee can mufflers. Cops don't give me a second glance, yet if I want to I can go into warp speed.
I can't speak to the car's handling prowess because I'm not going to hurl it into a corner with so much speed that I need to worry about breaking traction. I like having power but I'm not a teenager any more and I am not going to endanger myself by going nuts around curves. If I wanted to do that I would've gotten a Boxster.
The only knock I have against the performance is that the shifter's software can be a little slow. Occasionally I'll put my foot into it, and revs start to build but the transmission takes a couple of beats before going "oh, I'd better get into the act and downshift!" This isn't unique to the R-Spec, it's a known trait of the Genesis.