GSXR400's race in the same class as V-twin 650's here (SV650's), although neither of them are 100hp+ monsters.
As far as I know, the GSXR400 is more or less the same engine as the bandit 400, barring a few 'upgrade' parts that GSXR400racer will sell you (cams, crank, carbs, swingarm).
I've taken mine on the track once, and will be doing so again in a couple of weeks. I thought it was great. They tip into the corners real easy, and I got a top speed of about 180-190 on the back straight (which for me at this stage is about as fast as I want to go).
The only minor problems I had were changing down gears after heavy braking, with the engine then revving too low for the gear I was in - tended to get minor lockup and tail wiggle. If I can learn to throttle blip that'll probably go away.
My long term plan is to rebuild a gsxr400 engine for the frame, and keep the bandit as a track day only bike. I'll then upgrade my road bike, cos the bandit is too much fun to get rid of.
One of the reasons I'd be doing that is because the bandit is just a wee bit too small for me - I get bad leg cramps after more than a couple of hours riding. The SV650 is a bit bigger.
The only other thing you might want to consider is that parts for the SV are far easier to come by.
So, yeah. I think they're worthwhile.