The big(ish) V twins don't really qualify as supermotos, but they can be ridden like one, and they look 100% like a SM. But, a SM is about handling & absolute flickability. If you follow any SM racing at all, you know that the 450s are MUCH faster than the open class (singles up to 700cc), because of weight/rotating mass/etc. The best performing (road legal) SM is going to be a converted WR450/CRX/KX etc, whilst a factory SM (690SMC/530SMR/640&660SMC) will be very, very good, and reliable/street legal from day 1. You always trade-off reliability for performance at this level, so choose your poison. 
FOr more of a road-going (only) SM-style bike with street manners, the Duke II (I had 2) is a great used buy. I miss mine, until I hit a straightaway on the SD. 
As a hijack - anyone here own a 690 Duke and a Duke II? I wonder how different they are, as I'd like to get a big single back in the garage, and I'm curious to what someone who's owned both has to say........ 
TIA.  
