Hi,
The 250 Ninja is a great bike. I've had the opportunity to test ride one, and they're lots of fun, especially for a brand new rider - lightweight, decent acceleration, don't cost an arm and a leg, and they're pretty good bikes.
HOWEVER... they are still just 250's. If you get this bike, you'll have a lot of good times and great fun on it, but you'll probably be left wanting more after 6-9 months. They don't have the get up and go that a bigger bike has, and they bog down under 6500-7000 RPM. They have a high revving small parallel twin engine so they sound like a bee farting most of the time, and they don't have a huge amount of torque because... well... it's a 250.
Servicing is quite cheap, selection of tyres is quite good. At 60 mph (our freeway limits in Australia) it was revving a little but had plenty to go. Top end, compared to the Ninja650, is quite lacking, but you get that with a 250. Economy was quite good too - I milked around 280-300 mi out of the one I was riding before it needed gas (ran it on 95RON fuel).
They're small too - I'm 5'11"-6'0", and I was towering over the bike.
The Ninja 250 is a bike I would buy for a 6 month run on bikes before I upgraded to a new bike. Upgraded doesn't necessarily mean that I'd get rid of it either.
I don't have any riding experience with the ER-6F (Ninja650 equivalent in Australia) but I was looking at buying one when I started considering a second bike. They're light, easy to handle compared to some of the larger bikes, fun to ride but have a low top end compared to some of the other bikes out there.
I didn't like the delivery of the power, which like the Ninja250 bogs down a bit at the lower revs but really kicks in as you rev it out, and as such it fell off my radar for potential bikes. When i was asking around about people that owned them, they said they were great little bikes but sometimes left you wanting more. ECU mods and such help there.
One of the guys also said that they've got a weird buzzing rattle that happens at certain revs - adding the appropriate rubber mounts fixed his problem and cost him next to nothing. Apparently the exhaust note sounds pretty bad too, so most Aussie ER-6F/N riders switch the exhausts at the first opportunity they get too.
The ER-6F/Ninja650 is a bike I would buy after the Ninja250 if I enjoyed my experience on the 250.
Hope this helps. Both are excellent bikes based on well tested designs and knowledge, so either way you'll have a great bike.
EDIT: as fsubetcha mentioned, they're great in the turns. I was absolutely owning a ZX-12R through the turns because of the weight, but he was killing me on the long straights
.