Test rode some bikes! This is coming from someone who rides a Ninja 250R.
Honda CBR500R. Looks sporty, but when I got on it felt completely different to my Ninja. It made my 250 feel like a supersport in seating position. Tons of legroom, not much forward lean (or so it felt). Motor is really easy to use, decent torque for getting around the city. Really easy to maneuver at low speeds, and I think it might be impossible to stall this bike! Don't really have to rev it off the line at all. Didn't handle as well as my 250. Would have preferred the bars slightly higher. Nice bike all around, but I definitely understand the "sheep in wolf's clothing" quotes now.
Honda CB500X. No photo so stock image, but it was identical. Same engine as the CBR500R, but a bit taller, much more upright seating position, slightly shorter gearing, bigger gas tank, etc. The long and short of it is I think this is the best of the Honda 500s. Corners surprisingly well (in an upright way), you have a great view of the road, and the seat was extremely comfortable. Could definitely notice that this was a bit faster than the 500R (again due to the gearing). Great bike, unfortunately for me my back doesn't like upright seating. Low price. Highly recommended.
CBR650F (new for 2015 in Canada). Beautiful bike. I didn't plan on testing this but it was available so I figured what the heck. Has about 84 (87?) horsepower, so by far the fastest bike I've ever ridden. As soon as I started it up it oozed 'premium' after riding the 500s. Very good fit and finish. Seating position was almost identical to my 250, though higher seat height. Wonderful sound (inline-4), easy to ride at low speeds. Absolutely tons of grunt in most of the range. After about 4000 rpm (11k redline I think) it just pulls. Flattens a bit and then comes back on the power a bit higher, but I didn't push it much. Front end was noticeably short after riding the other bikes, which took a little while to get used to. Clearly not as nimble as my Ninja 250, but the bike was a blast to drive. Good seat but clearly not as comfortable as the 500s (in particular the X). Also, I was shocked at how quickly it revved when I went for downshifts.
I'm now split between wanting the nimbleness of a small bike (like my 250) and the comfort and power of a larger bike (like the 650F).
All in all, compared to my Ninja 250:
CBR500R -- not worth the upgrade as far as I can tell, loses the sportiness for little gain IMO.
CB500X -- totally worth it if you want something upright, comfy, and can handle a bit of off-road.
CBR650F -- lovely 4-cylinder, lots of power but not unpredictable, seemed like a quality product. not sure I'm willing to give up the low-speed fun of my little bike but when I was riding it I was talking to myself in my helmet, so yeah it was fun. :O