I've played in the Home Counties Premier League, the Surrey Championship Prem and the Middlesex Premier over the last 10/12 years - though this year I've moved back to my boyhood club in the Thames Valley Cricket League.
In my experience, without a doubt, the Surrey Championship is the best. The other two aren't a million miles away, but the players you get are all current or ex-first class counties players, and they aren't scared to bring in a high quality overseas or two either.
I think it's great that this cricket exists - but for some of the clubs too much focus is placed on the 1st XI and the rest can do whatever because the chairman doesn't care. It's a shame when it's run like that. I played for a club, no naming names, but when a first team player got injured, instead of calling up a 2s player, the chairman rang Giles White at Hampshire and got in one of their second string in. Pitiful.