Kinda like MM except instead of ranks you have RWS (round win share) and dependent on your RWS you play different skill levels. Almost every (NA) pro player plays on ESEA over MM because of
128 Tick Servers (MM is 64tick)
ESEA Anticheat (better than VAC because it's more intrusive)
League ruleset (35 second bomb timer and 1:45 round timer I think)
And probably many more reasons. It's just all around better. Your aim feels better, you play better players, and you rarely question whether a player is legitimate.
I've played against Tarik, ptr, and Hazed from CLG. nitr0 from liquid and even jdm64^v^ from SKDC. It really is a much better learning experience than MM ever was. However, the next step after ESEA pugs is scrimming. Which me and my team are still practicing at (currently 0-11 in scrims :S)
If you have the money (about $7/month) I highly suggest it. Here's my profile -> http://play.esea.net/users/743324