Brian at MOTYDesign originally started making the 4-Cell QD batteries for 600RR.net members and other I4 bikes. There are some members on that forum that still have is original 4-Cell design powering their bikes. They are small and very lightweight. I have his 8-Cell QD in my Honda and it hasn't missed a beat. It cranks stronger than with my OEM battery and fires right up.
Here is the weight comparison on my Honda battery and the MOTY.
I saw that everyone was talking about looking like it's just wrapped in black plastic. That particular model is, but it is still very high quality. Also, he does offer OEM style hard cased batteries as well.
12 Cell OEM Hard Case. And he also has them wrapped with terminals as well.
For the Buell, I just got mine back on the road after spending the entire summer trying to figure out why it would not stay running. Wolfo and I finally got that figured out so I just started riding it again last week so I haven't had a lot of time to see if the 12 Cell is adequate. I will try and get a weight comparison to night to post up. It is really light compared to the stock monster that's in there and takes up about 1/3 of the space.
As for my XB12R, the verdict is still out on it. The fan running is what has always worried me. So far I have put it in and the first start it bogged it way down on the first crank and on the second it fired right up. Went for a ride stopped and started it a few different times after 10-20 minutes riding each time. It would fire up on the first crank each time. The next time I rode, after sitting over the weekend and having the fan run the last time I shut it off, it bogged down a little (about as much as my stock battery did) but fired right up. I rode for about 30 minutes before I shut it off (the fan ran). Had it off for a few minutes and tried to start it and it bogged way down like the first start with the new battery. After that the bike was off for about 30-45 minutes again. Went to start and it started up easily very little bogging.
I got my 12 cell from Brian on a trade for my stock 600RR exhaust so I didn't pay anything for it. After being impressed with my 8-Cell in my Honda, and other work and dealings with Brian with my RR, I was going to purchase the 12-Cell for the Buell before we worked out the trade. Overall they are very high quality and probably the lightest battery out there. You won't find a better person to deal with than Brian. He has a strong and longtime presence on the Jap bikes forums from his gauge work and batteries. I think these batteries are perfect for those bikes.
Our riding season is about over here in Nebraska so I won't get a ton of time with the battery in the Buell before I pull it for the winter, but so far it is working (better than my dying OEM was). Not sure how all the the other lightweight batteries perform with the XBs so I can't compare with them.