Its been a while...
As the owner of two 'On-Ones' I feel pretty well placed to have an opinion here. Bike #1 is an early Inbred slot drop out singlespeed, fully rigid. Bike #2 is an '09 456, 120mm forks.
They're both great bikes, I've completed the HONC on the Inbred in perfect comfort which is a pretty good trial for an XC bike. The forks on the 456 are 100-120mm adjustable and with a little tweaking of travel, the bike is amazingly versatile. I've always found it to be very stable and confidence inspiring on descents and it climbs better than lots of other bikes I've ridden. My riding buddies said that I was noticably quicker on the 456 than I was on my previous bike... that says it all. The inbred, despite its age (its pre-disc mount) and reputation, still has all its original paint
The bikes pose many people a problem because they are a 'different' fit to many other bikes out there. Follow the companys guidelines about stem length and bike sizing/set up and you'll have a cracking bike. Ignore it and you'll hate them.
I would also add that when I received my 456 I got the bottom bracket and head tube 'chased and faced' by the pros at my LBS and I put some rustproofing through the frame before building it. Even factoring that in, the frame still cost less than £175.
I have no hesitiation in saying that my Inbred and 456 are great bikes, but then I took the time to set them up right and have adapted my riding style to get the best out of them.
I'm watching the development of the carbon 456 with interest and will be seriously considering getting one. If it rides with the versatility and ability of a steel 456 but weighs a pound or so less, its going to be a monster!