Picked up a used Sir9 off here. Really like it. Relatively light for a steel frame. Ridden it both rigid and with a bouncy fork. Really like like it in either guise. Tend to swap the fork from time to time.
Bit lighter with the rigid fork obviously. Rode Mountain Mayhem with it single speed and rigid and it was very good – other than the final descent in to the stadium where it was screaming for a fork.
Put bouncy fork and gears on it for sleepless this year and it was superb.
Previous experience is of an inbred 29er which I really liked. The niner is 1.5lb lighter, steers a little quicker. Frame feels a little springier and possibly slightly more comfortable (though tyre choice probably makes more difference).