I expected Free! to be some sort of male version of a bishoujo series: pretty boys (in swimming tunks!) doing inconsequential things. But it turned out to be something else. It has some elements of a sports series: three boys begin a swimming club and dream of reaching the national tournaments. There is even a rivalry with another school!
But it's all very character-driven, which is unlike most sports series. The rivalry began a long time ago, with all sorts of old stuff going on that adds depth to the story. And the character stuff is much, much more important that the sports stuff -- there's no "burning youth!", it's not shouty. Instead, it's about the complex relationships between five boys who met each other through swimming -- how they developed and how they resolved their rivalry.
But it's not heavy-handed at all! Indeed, the tone is light-hearted and funny, and the characters have been drawn (and voiced!) with great care. It's easy to watch, but the depth of the characters make it much more than just a throw-away series.
I'll give it an 8.