I have the European (and Canadian) Ninja ZZR-250, so I can give some input.
It's actually my second bike (my first was a dualsport to get some dirt experience), and I think it's an awesome choice...
First of all, you have to realize that on the street you can only go so fast and push so hard before things become EXTREMELY dangerous. Most of my riding is twisties, and I always ride with 600s, 750s and 1000s. I can keep up easily, and I'm faster than a lot of the guys on the above bikes through twisties. The handling is just sweet, and I more than double the recommended speeds in the turns, I usually do 60 km/h turns at 120-140 km/h for example. Sure, there's a few guys I ride with that rail 60 km/h turns at 200 km/h or more and have completely scrubbed to shit front and back tires just from riding on the street, but think of how dangerous it is going that fast. What if there's sand in the corner, or a rabbit, or a deer, or a car in your lane in a blind turn, or what if a car pulls out from a sideroad because they don't expect your little headlight to reach them in 3 seconds because you're going 240 km/h on the posted 60 km/h road. I still get up to 165 km/h or so in the straights, and believe me that's plenty to kill you if you fuck up even a bit.
The problem is that, in the twisties, I'm keeping the bike at a good 10-11 + grand rpm in 6th gear for half-hours at a time, which is probably unhealthy for the motor. Basically, you always have to ride twisties in 6th gear, wide open, ALL THE TIME, unless they're the very slow ones (turn speeds posted at 20-40 km/h). Then you do a lot of shifting and learn how to keep the bike in the right gear. You'll really learn how to shift that way.
My complaints are: the suspension is too soft, and twitches in high-speed corners. The bike absolutely SUCKS for freeway riding. You're going to want to keep off the freeways at all times. Even riding the secondary highways you have to keep it in 6th gear at 7-9 grand just to cruise to the twisties, and then in the twisties you have to keep it pretty much wide open all the time, unless they're very slow twisties. Also, I'm not really happy with the brakes... in stock form they're not NEARLY as good as those on 600 cc + bikes. The stock tires aren't great either.
If you get some skills, you'll have no problems keeping up with average 600 riders in the twisties. For now, that's all you need IMO, because a newbie slip like too much throttle in the middle of a turn won't send you into a cliff as quickly. Make no mistake though, you can ride this bike FAST. It takes a while to get up to speed, but you'll learn SO much about keeping your speed up through corners, because on a 250 if you lose speed, it takes forever to make it back up after the corner, and your buddies will rocket by you. Get it up to 100 mph and just try to keep it there. Quite honestly, I'm a little scared of how fast I'm going to be able to ride a 600 when I eventually move up, because acceleration wise, the 250 is utterly gutless in comparison, especially at those triple-digit speeds.
Power wise, I was bored with the bike the first day I had it. You can keep even with a Mustang or Subaru WRX up to about 60 mph if you race from a light, and then they really blow by you like you're going in reverse. If you slack off pulling away from any light, you're going to hold up regular city traffic, and I'm not kidding. Launching it at 10-11 grand rpm though, can take most street cars off the line and to the next light.
Basically I think that a beginner has no need to go fast. You need to learn to ride without getting yourself killed, and then you can worry about the rest. This bike is great towards that purpose. Additionally, it'll teach you to ride fast, BECAUSE it's lacking so much in the power department. You'll learn how to get up to speed as quickly as possible, and learn how to keep it at speed and lose as little speed as possible through turns.
When I upgrade to a 600, I'm going to do it because I prefer the looks and I'll have fun giving it little blips of throttle for shits and giggles, showing off for chicks, and doing wheelies and stoppies, but I don't see how you could use it to its full potential without ridiculously endagering yourself and others. If you want to see a 600 used to its full potential on the streets, watch Isle of Man TT, but insert cars, sand, gravel, animals, etc. into the picture... you can appreciate how retarded that is.
Good luck and have fun, whatever your choice of bike will be.
