There are 5 On-Ones sitting in my a garage, so you could say I’m a fan.
There are certainly lighter frames, but I love mine and have no real desire to change. Given that I stopped riding a Nicolai, an Endorfin and a Turner in favour of the On-Ones (gradually – I didn’t suddenly wake up and have a ‘must ride steel hardtails’ moment), it wasn’t just ‘cos they’re cheap, it’s more that they ride really nicely.
The Ti456 is lovely….if I had to pick just one frame to do everything, it might well be that one.
As for quality – the paint chips reasonably easily, the frames rust if you let them. That said, the oldest of mine is now 6ish, and rusting hasn’t been a problem for me – just a few spots here and there. The welds seem OK, and the only thing that killed one of my original frames was a freak stick/wheel/grunting uphill moment which just wrecked the rear end. Could have had it fixed, but it was cheaper to buy a secondhand frame.