I played quite a lot (50-100) of them ($10-$20) at one point and i managed to win about 70-75% of them which is about a 35% ROI but i would estimate my true ROI is probably closer to 20%. I would also expect them to get much harder from the $50 level upwards.

HU play does really improve your hand reading skills however so even if it wont make you rich playing a lot of HU games would def make you a better player.