Like the ones here:-
http://www.bike-mailorder.de/shop/MTB/Singlespeed/Trickstuff-Exzentriker-Exzenter-Innenlager-fuer-Singlespeed::10589.html

Looking at them as an alternative to an eccentric eno hub (no need for wheel rebuild etc) & a neater solution than a chain tensioner.

Just wonered if anyone had used them?

Yup, I’ve got one. It’s an elegant idea which probably could do with some improvements.

It won’t work with a MTB triple – the granny ring mounts interfere with the external part – even when I ground them off an old crank 🙁

So I got the cheapest road HT2 double I could (a Sora) and that’s fine, although gauging how many spacers is needed is a balance (on my frame – an XTC) between a crank rubbing the chainstay and not enough axle for the non-drive crank to clamp on to.

It works better than my Surly chain tensioner when the going gets really muddy, and after all that’s usually when I ride the SS as it keeps me lazy on the cleaning. For other conditions, and if you don’t mind a potentially vulnerable part, a tensioner is as effective (and cheaper).

drain has pretty much covered it.

I have an Excentriker and they are OK, they do the job but are a bit of a faff. Not as much as a dropout mounted tensioner though. Rather expensive but they aren’t exactly selling in big numbers so that’s to be expected.