Hm, i reached lvl30.
Conclusion: this game is really good if you are playing with your friends. We started using voicechat too, and have quite high win% in my opinion. However, if i try to play alone, the matchmaking system takes its revenge on me, trying hard to push my rate back to 50%. So i think i'll stop playing solo completely, and stick to my friends all the time.
My favorite heroes are Ashe and Zilean, playing the role of support or second-carry.
Also, we recently came up with the combo of Jax - Soraka - Corky - Zilean, and won 2 out of 2 games without trouble even after our 5th random teammates left both matches.
Something must be wrong with me. I'm giving advices to someone i should kill.