I've gotten a couple of half-day trail rides with the Syntace HiFlex. Not a cheap addition, but I'm sold on it.
• Seems to provide just enough dampening (so far – long days/multiple days will be the true test). I can't say that I feel it flexing, but that's what I was hoping for
• The clamp is quite long, and although not setback beyond a few mm, provides a lot of F-B adjustment latitude
• Metered markings are handy if you use a QR collar/drop it
• Lightweight at 231g (27.2 x 400)
• No weird shapes to impede using a seat bag or post rack collar
• Can be trimmed in length to suit
I'm using it on a steel El Mariachi SS with 100mm squish in front and carbon AM-rated hoops.
http://www.syntaceusa.com/index.cfm?pid=3&pk=2033