I had to put in some time watching matches and playing against bots. I didn't really grasp solid item use and maneuvers until hour 20 or so.
That said, after the fact, it was time well spent. Winning and pulling of successful ganks/moves has never been so satisfactory in any other competitive game.
Also, with Dota 2's ELO matchmaking, you'll spend some time with same skill level players, learning and teaching each other. Since it went more public this summer, everyone is a lot more cooperative and is willing to teach new players.
Additionally, you can open Steam's web browser to dota2altab.com to show you recommend skill and item builds for heroes!
My time with LoL was limited, but it pales in comparison to what Dota 2 has to offer in terms of competition, in my opinion.