I've had my MKIII about two weeks now. I've done some well-known rough testpiece type rides on it already, the most well known being Mormon/National at South Mountain in AZ (twice), also did Charoleau Gap in Tucson, known as a xc climbing training/grunt/hell ride, on the way up, and a pretty serious drop, rut, steeps filled downhill on the way back down.
The first time down National was the day I picked the bike up from the shop, I only had about 85 psi in the rear, 75 psi in the IFP, and the thing was so plush it was sort of disorienting, I couldn't really tell how fast I was going, and it felt like the rear was moving so much that it seemed to be ??floppy? like spagetti, I can't really describe it too well, it was only after that I noticed I didn't feel any sharp edged bumps or any harshness. It was just that the bike was reacting so fast and smoothing everything out so much it just felt wierd, compared to my trek fuel, which would soak up bumps but you would still feel the impact. I've since done a better job of checking the sag, setting up the damping (rebound was way to high, contributing to a pitch me over the front type of feel on bigger drops), and have settled on about 100psi in the main chamber for my 200lb riding weight (with gear, full 100oz camelbak, etc.) this gives me 1/2" of sag on the stroke, putting it right at the 25% that iron horse recommends for this bike. They finally have a little set-up recommendation now at
http://www.ironhorsebikes.com/homepage_links/news_events7.shtml though it doesn't mention the IFP for the air shocks, I imagine it should be at it's minimum also, like the CV/T of the coil shocks, but the listed minumum for the 5th element air's IFP is 75psi.
Anyway, after getting the suspension set up correctly, I could still fly down National, but now the bike feels much more composed. Still feels like I'm floating over stuff, but it feels like I can just hold any line. When I mess up and get off my line it still just holds tight and tracks true anyway. I would have to say the rear end is really stiff. This my first long travel bike, but it's much stiffer than my fuel. I'll try to post a more comprehensive review soon (sort of inadvertently did that here), but I'm liking it so far, climbs well too. Stock seatpost (not an EA-50 as stated on the website) sucks though, won't hold it's angle adjustment, always rotates back on a small G-out, a bump, or sometimes just going over a rut on a climb, torqued it to the 12 N-m on Easton's web site, but after a few slips, the teeth are chewed up now and it's shot. Oddball size at 30.0 also, no one seems to stock in that size, it's "usually ships in two to three weeks". I'll be trying to warranty it though, especially since it's not even the seatpost that was specced, nor is it a comparable equal, it's a "we're too ashamed to even put a model number on it", bottom of the bucket easton post, not the same seat clamp design as the EA50 and EA70 either, probably why it sucks. But I like the rest of the bike.
