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Had an 06 XB12SS. Loved riding it, but my ballz got the best of me and I traded it for a 05 GSXR1000. The outright acceleration of the Gix is always a rush, but overall the Gix was BORING to ride compared to the Buell.
After a number of other bikes, I'm back to, you guessed it, a Buell. I now own a 00' M2 Cyclone. I opted for a Tuber over an XB because the Tuber (as was mentioned) uses the HD 1200 engine with uprated cams, heads, etc (main point being the cases match the Harley motor). The XB uses unique cases along with the goodies mentioned on the Tubers. I was concerned that long term part sourcing may be more of a problem for the XB than the Tuber.
Test rode an 1125R with a D&D. That bike was fast. Fast in a GSXR750 with more torque kind of way! No idea about the long term reliability though. I'm sure, being a Rotax built motor, it will be durable, but it definitely requires more maintenance than either the Tuber or XB.
To their benefit, the XB and Tuber use hydraulic lifters = no valve adjustments. Single carb/throttlebody = no syncing. Belt drive (Tubers are adjustable, XB and Rotax are constant tension) = no lubing, no possibility of broken chain through the case. The XB and Tuber also have separate gear boxes from the crank case = no need to run expensive motorcycle specific oil; high end automotive oil for 1/2 the price or less is perfectly acceptable as it won't touch your clutch. Fuel mileage is also terrific compared to other sport bikes (my M2 gets mid 40s during commuting and around 40 when I whip it, the XBs are even higher at about 50 when commuting!)
Overall, I think the XB and Tubers remind me of older Corvettes compared to newer sport cars. They can be made to rip down the 1/4 miles but likely will always fall behind for terminal speed. They handle well up to 8/10ths but need much work to be comparable to a SS bike on the track. If that is what you want from a bike, you won't be happier.
If you're hoping to get something different that will keep up most of the SS bikes out there, I'd recommend grabbing a Rotax Buell, a Tuono or a Speed Triple.