A local year round messenger that I regularly chat with at the LBS was a prototype tester, and loves his. He's had no problems with it at all. I gave it a quick test ride (IIRC he said it had about 10,000 km on it) and it shifted surprisingly smooth.

If you have any specific questions please ask. I see him regularly and can hopefully help to answer them for you.

Ah ha, thanks for that.

I keep getting drawn to the concept because it widens my riding envelope dramatically and when you live in a hilly area like I do, that's got to be a good thing.

At the moment, my Hillbrick has no gear cable guides under the bottom bracket (or anywhere else) - just like a track bike. I'll need to work out some way of running the cable under there but was thinking that simply clamping a piece of cable outer in place would work for initial trials at least. Any thoughts?

Zip ties for sure.

I like my S3X it gives me,with a 48x15 gearing with a middle gear of about 66 inches.I spend 80% of my time in this gear.Top gear is about 88 inches useful for swooping down hills and a low gear I have for seated climbs.Trouble is it keeps slipping in this gear,which is annoying to say the least.Oh and of course the old 24 inch gear.In my opinion the S3X is a great idea,but with a teething problem.

Can you expand on this? What is the 'slipping' and where is it occurring?

and while I haven't rode it a ton, it was in semi-regular use for about a year. Mine is one of the original BD pre-orders that I splurged and had a custom frame made for from an Ebay auction.

For my commute the first 3-4 months after I got it, it was my regular ride. My route dropped almost 400ft in about a mile down to the waterfront, then up/down a couple small overpasses to my old job. Some days I would climb ~100ft in three blocks to play squash before reversing my route back home.

I typically run ~63 GI on fixies, which was the best balance between some semblance of speed and my ability to ride the to places that I needed to be. I have a disc brake up front and a V in back and they both get used. My S3X is 42/17 for 66/49/41. Those extra 3 gi are enough to keep me semi-comfortable descending overpasses. I'd prefer a wider spread, as the 41 is a little close to the 49.

I haven't had any problems with mine and I know its lost a small amout of oil because I remember it spurting out more than once when had to remove the indicator spindle to install a freewheel during knee injury.

I just built up an S3X here. I used a regular Sturmey-Archer cable pulley to route the cable past the bottom bracket:

The slipping I am talking about is really 1st gear jumping out of gear into a false neutral.I certainly wouldn't risk doing a standing climb in 1st gear due to there being a genital, top tube interaction.