As taste is a personal thing, I won't comment on looks. I also own a 2008 Ninja 250, so I'm going to be biased, and when I move on from this bike, it won't be to another 250. The CBR 250 will most likely never make it to my garage.Having said that, in my opinion, having owned the Ninja, the CBR has big shoes to fill.
For a start, the Ninja is a tremendous bike for such a small package, and its biggest asset is its flickability. Its manouverability is awesome, and I've ridden bigger bikes on a track, so it's fair to say I know the difference. Not sure about the UK, but in the US, there is a helathy number of individuals in the US and south-east Asia racing the 250, or at least taking it regularly to track days.
I have done a 3000km+ trip from the north of France to Italy on the Ninja, and will tour Northern Europe with this same bike in May, because I know it has the oomph to take me where I want to go. In fact, it had what it took to ride the Autobahn to Frankfurt, with luggage, without any trouble (of course, I wasn't breaking any speed records). I would seriously like to try the CBR, not to consider it as one of my own, but to see it if truly is a 'real rival'.