If you're unfamiliar with the gameplay, it's a hero-based RTS wherein your team of heroes (or champions) tries to kill the opposing team's buildings while they try to do the same to you. If you understood the jargon there, it's really not any more complicated than that. The "free"-ness only extends so far, however, as they obviously want to get some money out of you. But you can play almost comfortably with just the free currency you earn by playing and winning matches. Otherwise, you can buy points to get more playable characters, runes, etc.
Anyway, I'm having a pretty good time with it. There's some good tutorials in place and you can play against bots (for points) to practice up and try out new champions before going against real people. I've heard some horror stories about the community, but so far I've not had any problems at all (except for people with "Penis" in their nickname. What's up with that?). There's also a refer a friend program too, so if you want to try it out you should click on the link on my sidebar to join up! My nickname is MikeyH, friend me!
So yeah, I'm still not sure if I'll be picking sides with the whole Blizzard Dota/DOTA2 debate. Blizzard Dota will be free, so that's awesome, but it'll probably have less support and added features later on as well. DOTA 2 will be out sooner with more support and will probably be taken much more seriously in the esports community overall, but I hate how they stole a lot their character models right from Warcraft 3. That's kind of cheap. Either way, I'm sure they'll both be lots of fun. Here are some videos for them too:
I may have already posted this trailer here, but I love it so.
I wasn't able to find a trailer that had real gameplay in it, so here's a short gameplay clip of DOTA 2.