A quick preface to this review: I am 6'1" (185 cm) and 220 lbs (100 kg) without riding gear. This is my 8th motorcycle, and previous bikes ranged from 650cc to 1100cc, BMWs, Kawasakis, Moto Guzzi, Honda. I have been riding since 1999, every day from 2003 to 2006.
The Ninja 250 has ended up being essentially my perfect bike. I had to make a few changes to accommodate my comparatively heavy weight: I replaced the fork springs, added a Gold Valve Emulator, and replaced the front brake line with a stainless steel braided line. I replaced the rear shock with a Hagon unit, with a spring correctly sized for my weight. I replaced the tires with Bridgestone BT45s (although this modification should be done by everyone -- the stock Dunlops are terrible).
The bike does everything I want it to. I can ride on the freeway (getting up to about 80 mph before I have to worry about whether I have enough power available), on long trips, through the canyons, and around town. I get between 48 and 70 MPG. I can out-accelerate any traffic I need to. I took it on the track and spanked a couple of SV650 riders, who later commented that, as previous Ninja 250 owners, they'd had no idea it was capable of such performance.
This bike, however, is definitely not for everyone. It doesn't have the kind of mind-blowing power a CBR or SV will give you. It doesn't do well with a passenger. It will not get appreciative catcalls from hot women (unless they just like the fact that you're on two wheels).
What it does do is deliver enough speed and acceleration to get you a speeding ticket in any jurisdiction, while cornering precisely and easily (it only weighs 304 lbs dry, after all), and returning fantastic gas mileage. It doesn't require excessive maintenance, although it's not as maintenance free as some bikes. It uses cheap tires, and doesn't tear through them. With a few inexpensive modifications to suspension and brakes, it becomes a perfect motorcycle.
And it does all this at a new price of US$2999. Used models in this year should be available in a safe, rideable state from $1700 to $2500 depending on condition (as of 2009). My current insurance cost is $17/month for very good coverage.
I recommend this bike to anyone who wants a motorcycle that can do it all on the pavement, and who doesn't have something to prove.