I've only played them at the lowest stakes ($2 / $5). The strength of the opposition there can vary dramatically and even against the bad players you've got to be very good at changing your game and adapting - they're not like STTs or full ring games where (at least in the low stakes) you can adopt a standard strategy and just grind it out because a strategy that works against one opponent HU will be suicidal against another.
It's doable if you're committed to them and good at HU, I wouldn't say it's some untapped gold mine or a sure-fire way to build a roll anything though.
I tried these for a while and the variance was ridiculous. Also , at the micro-stakes, the rake is so high that it really takes a high win rate to come up with a decent profit (I happen not to have rakeback but if you do its obviously easier)