Fan-freakin-tastic! This bike absolutely blows away anything else in its price range in just about every category you can imagine. Twin cylinder. Watercooled. Fully faired. Disk brakes front and rear. More power than other 250s and even some 500s.
And I love the look - I bought a Candy Red 2007 for $3400 out the door.
So far, I have been very surprised that this "little" 250 has no trouble keeping up with traffic, and as long as you keep the engine in its powerband (6500+ RPM), it has no trouble accelerating out of most situations. The 0-60 on this bike is listed at 5.7 seconds, which is 2/10ths faster than a 2002 Porsche Boxster S. In other words, it's plenty fast.
While you won't be pulling any wheelies on this bike (which is a plus for safety-minded riders in my book), the engine has amazing power for a 250. In fact, it is listed at 36hp which is, by comparison, nearly 75% more than you would get on a fuel-injected 250cc Piaggio. It's close to the same HP as some of the 500cc cruisers.
The bike is also very light, weighing in at 304lbs dry and 355 "wet" which is a very light bike indeed. You feel this weight, or rather you feel the lack of weight, riding this bike through twisties where it handles like a dream. It's very easy for beginning riders and very forgiving of beginning rider mistakes. The light weight also helps it avoid the dreaded "top drop" where the bike falls over at low speeds (say, in a parking lot) because the center of gravity shifts and the rider isn't strong enough to keep it from toppling.
The riding position is also good, but it's a strange middle ground between a leaned-forward sportbike and a leaned back cruiser. Most of the time its great, but it more closely resembles a standard motorcycle position than a sportbike position. For me, that's a good thing.
There are two downsides to this bike, although one can be forgiven considering that this IS a 250. The first is that the bike has very low torque and if you get caught driving around 3500 RPM and need to suddenly accelerate...well...you won't. This lack of low-end power is normally not a big deal considering how easy it is to keep the revvs up, but it is something to be aware of. The second is the front suspension nosedives pretty steeply if you get on the front brake, and if it bottoms out you risk a front wheel lockup. This is easily fixed and is less of a problem for lightweight riders.
Its fully rideable in all situations. I've cruised at 85mph on the highway, blazed though back canyon roads, and rode around town. It does all with aplomb. In fact, I would argue that its more fun to ride te Ninja than most sport bikes because you can really use all the gears and RPMs while remaining at sane speeds!
Mileage varies based on how you ride, with highway riding being the least efficient. I get an average of about 60mpg.
A guy at work who rides a BMW F650gs (single piston thumper) was constantly giving me grief about my "little" 250. He stopped when I pointed out that my bike cost $4,000 less than his, accelerated faster (5.75 ninja vs. 5.9 BMW), and had the same top speed.
If you're looking for a great bike, you can't go wrong.