I ran 1x9 on my hardtail at the start of last year just to see what it was like. Now run it (properly with a guide and SS ring) on both my bikes. 32t up front and 11:34 cassettes.
I tend to rarely venture out of Calderdale these days due to family commitments (and the fact it has the best riding in the world) and haven't yet found a climb I can't do on this setup vs my old triple/double setups. Of course there are climbs around here that I couldn't do but I don't do those anyway - easy climbs and mad descents!
Only place its come unstuck was at Kirroughtree the other weekend - the steep climb after McMoab and the other mad steep climb near the end. I was walking both those but they're horrific with a granny ring so not bothered about that.
One thing I will say is that I was a long time non-believer of running a top only MRP/e13/Superstar style guide. But after a night ride the other night with one of Superstars XCX guides and a proper SS ring (On-One stainless jobbie) I couldn't quite believe the chain stayed on. Reccommended... Only running an MRP G2 with tacco on the full sus as I tend to ride that thing 'into' stuff!
A lot of 1x9/10 depends on the riding you do. Wouldn't want it if I lived in the Lakes etc.