It's not amazingly light, 1300g-1400g, the website quotes different weights in different places. if you wanted to save weight you could probably buy a scandal for £200-£300 less and spend the change on lightweight stuff you wouldn't otherwise have done.
One of the guys I've ridden with at Swinley off here has a kinesis maxlight pro 3 (the model on the STW homepage), if you want a good looking, well built, lightweight frame that rides well and not particularly common then its definately competition for the on-one.
I've only ever ridden carbon full suspension bikes, and they do feel a bit 'dead'. But thats a combination of the design, the material and probably to an extent, my prejudice.
On-one as a company is fine, never heard a bad word about them if it breaks and its their fault you'll be covered, if you crash it and break it then they sometimes do crash replacement frames at a reduced price.