You can't really fault The RD's, they were always a great bike. Based on a racer, they had all the right stuff for a motorcycle, light, quick handling and good power for their size. Great for canyon carving and perfect for a teenager getting into street bikes.
Kawsaki wanted a "moniker" motor, and went with the triple, when the prototype 500 rotary twin made the same power. And the 350 rotary twin were actually a bit faster than the 350 triple. A bit of fashion over form. I personally think twins are a better motor for 400cc's and under, the weight and width of a 3 cylinder motor is a bit much for that small of a capacity. The S2 is a cool bike, and the "rush" JP likes is great fun, but for racing, that is a hinderance. Reed motors have more power down low and smoother power curve, that makes for better racers.
But with the EPA and the civilization of these little bikes, they lost their some of their "POP". With both the RD 400 and S3, they got milder, longer and heavier. And frankly, that sofa lounger seat on the 400's ruined the looks of the bike. (but comfy they are) Still great fun, just not as much as earlier models.
I would just suggest riding it and then see if you like it. They really weren't made for 200lb. old farts like us, but that never stopped us!
My first "real" street bike was a 500, so that is kinda my comparitive for "fun" power. The smaller bikes are a bit lacking there. At least stock.......
