I was waiting for Piers to respond first, because I was very curious as to what he would say....In my limited forays into 2/4 and 3/6, I could not agree more...The 1/2 games are just brutal these days, perhaps inhabited by the BW's?? I really don't know, as the games are beatable, but the BB/100 are VERY low for me....FWIW, some of the other sites and blogs are essentially saying the same thing...2/4 and 3/6 is where the money is...The full time BW's (if in fact this is the problem) are not really a threat, but the games are generally tight, and even the true "fish" seem to sense it, and tighten up as well...
2/4 if your BR will allow seems to be the answer...

I have never played 5/10 online (yet)........

GL

From what I have seen I agree w/ Piers. At 5/10 I see a very noticeable jump in the quality of player.

And at 10/20 (not many hands) i felt out of my league.

I found the $3/6 game to be a bit more aggressive than the $2/4. But my sample size is small.