Hmm...
Well I guess you havent sat on a Ninja

...
The 250, 500 and 650 Ninja's are all upright...
I have not done a back to back on them with the buell but I'd atleast sit on one because they are both very similar... The new 2008 Ninja may be a bit sportier but I have heard its still upright, but the 2007 Ninja I sat on fit me better and felt lighter then the buell (which is supposed to have mass centralization and all that)...
Lastly there are probably better bikes out there then because the buell blast IS very close to a sports bike... It is a nice blend of metric cruiser and (american) buell sports bike features... Not to say its not a good bike but it is probably one of the worst performing 500cc bikes out there...
Lastly, while I agree you dont want to be on the freeway for atleast a month or two, statistically its safer then being on the local 2 lane mainstreet and having grandma pull out in front of you...
I'd much rather just have to worry about cars changing lanes into me then the left handers and other issues you see on the "regular" streets...
Good luck either way with your purchase!
And if you want to give the Ninja a little chance, go on ninja250.org, in the FAQ they have a nice article comparing it to the Buell...