On this board you'll get much praise for starting with a 250. But I would have been very dissapointed had I started on a 250. Sure it's a great learning bike, but if you're going to keep it more than a few months, you'll probably want something bigger.
Then again, if you've never tasted the power of anything bigger, and don't know what you're missing, you may not feel the need to upgrade down the road.
I started out on a CBR600 F2, and it was a great learner bike IMO.