My first bike was a fully rigid 2003 Giant Boulder. After much deliberation in shopping for a new bike, I chose a Niner Bikes SIR 9 because it can be configured as either single speed or geared, it has 29 inch wheels, and it has a lightweight steel frame. I bought it in Virginia, where the single speed characteristic worked okay but some of the mountain slopes were too steep and too long. I live in Iowa now and it feels like the perfect perfect bike for this area's rolling hills. My LBS in Virginia put the bike together using a mix of components they had around the shop so it is not a standard Niner build.
I am huge fan of the single speed configuration because it is helping me become more aggressive on hills. Specifically, I now spend my mental energy on pedaling hard rather than shifting gears. I have a 32T crankset and 20T rear cog and, so far, this gearing is working really well. The gearing is conservative enough for me to be able to climb the hills on my favorite trails while at the same time giving me enough power on less aggressive terrain.
The following are components that my LBS chose for me because, honestly, I don't know much about them and wouldn't have known what to choose. This is my first bike to have any of the following: tubeless tires, front suspension, and hydraulic disk brakes.
My SIR 9 frame weighs 26 lbs and feels really light compared to my Giant Boulder that weighs 34 lbs. I chose the SIR 9 over both the Gary Fisher Rig and Kona Unit primarily because of all of the great reviews I read online regarding the Reynolds 853 steel tubeset. I am not a mechanical engineer but have bought into the steel is real philosophy.
On my test rides, the geometry of a SIR 9 felt much better than did that of a Kona Unit. The Kona Unit felt too compressed from the handlebars to the seat (maybe it needed a custom fitting). The geometry of a SIR 9 feels so natural that I often forget that I am on a 29er instead of my old 26er.