I've been beta testing League Of Legends since early July of 2009. What can I say, I loved Defense of the Ancients (WC3 Custom Map), and I had to see how well this game related. It really is a beautiful game, the graphics will please you regardless of what system you run (as long as you meet required specs). Best way to categorize this game? Session based RTS/RPG hero team fights and continual summoner progession.
This is a fast paced game that will always keep you on your toes. The summoner progression and character progression will literally keep you striving to better yourself for as long as you see fit. You could literally play this game for 5 years, and not get bored. For example, I played DOTA since the Reign of Chaos DOTA maps came out, and continued to play the game for about 6 years. I replaced DOTA with League of Legends, and I do not regret my decision one bit. This game is so good, I even got my wife to play it! She never plays computer games, and she loves LoL (best acronym ever for a game?).
Another thing that puts this game "leagues" above other free games, Riot Games will not SELL gamers advantages over others! I love this, I've played too many games where you can simple buy your "power" or items, and stomp on players who do not have the money to compete. The only thing Riot Games sells in alternate character skins and other things that don't really give other players unequal advantage.
I encourage everyone to try this game for free, and if you really like it, please, PLEASE, support the developers and buy the game guys, they're doing really great things, and come on, they built the engine from scratch! Hook 'em up!