I learned on a Honda CB350. It was 10 years old when I got it. Much larger than the Ninja and 375lbs wet with 36 hp. Great bike to learn on. Easy to work on. Easy to ride. I miss bikes being big enough to be comfortable all day in stock form.
The 250 is probably the best bike around these days for learning and then still having something that is rideable and very enjoyable after then initial scared to death learning stage. It will keep you occupied, as has been said, for quite a while learning to really ride it instead of just going down the road.
I've been riding for longer than I care to remember. Funny how time just gets away from you. I've ridden everything from the little honda 50 and 50cc mopeds to liter bikes. I just bought an 05 Ninja 250 last year. I'm having a blast on it. You can do things on it that you will never do on bigger bikes. Twisties are wonderful on small bikes.
Small bikes will do almost anything you demand of them. They will ride thousands of miles non stop except for gas. They will run a mile to the store. They are easy to work on. Parts aren't too bad in price or ability to find them. They hold their value really well.
If or when you decide to get something bigger, you can sell for near what you have in it. In some cases, more. There's always a demand for learning size bikes.
The FI would be nice. No irritations with big temp swings or altitude changes like with carbs. But, you have much more electronics to fail and they are quite expensive if you have to replace them.
The 250 is rock solid. You will enjoy it. Have fun. Learn. When you think you've learned all you can...then you can really start learning.