I use RockShox MTB Post. I ride a lot (marathon, xc racing) and it helps a lot. I've bought it because of back problems (pain) which has stopped from finishing few races. It does well as for a seatpost. It has some play, which is annoying, but helps on flat, but rough terrain, when it's good to stay seated but its impossible on a regular hardtail (roots, rocks). There are some problems with setup - when i use softer bumper it work while pedaling, wasting my energy
. When i use firmer one, it sometimes expands to fast, and my ass is jumping slightly on saddle. I prefer the second setup, as 'jumps' don't happen very often, and are not dangerous and there is nearly no wasting energy. The problem is that my weight (67kg) fits both bumpers and no of them is really good.
Maybe one day i gonna cut it.
Of course, seatpost is not half as comfortable as FS, but when you dont have $$, or dont want a FS it is an option.
There's one with remote adjust on hndelbars, including locout - it may be worth checking.
Don't remember the manufacturer. Maybe Easton... dunno
Cheers
yash