Summary: This is a great game, with several innovations over tradition d20 games that make it quicker and more immersive. That said, the classes are not equally complex or customizable, and the game depends on joint collaboration and story-telling on the parts of the players and the GM. If you like hard and fast rules, and detailed combat/tactics, this may not be the game for you. On the other hand, if you want to play a fast-paced game that emphasizes flexibility and creativity, and your group is OK with some DM fiat, then this is the game for you. Highly recommended.
Detailed Comments: I ran 13th Age at GENCON earlier this month for two groups who had never seen the rules before we started playing, and I have also played it with my home group. All three groups enjoyed the game, found it easier to pick up and play than other d20 games like DND and Pathfinder, and particularly enjoyed the flexible backgrounds that replace the traditional d20 skill system, the relationships the characters got to choose with the 13 icons (positive, negative, or conflicted), and the "one unique thing" that each player created for themselves that differentiated them from every other character.
It is quick to create characters, players choose how they want their characters to be connected to the game world (and these choices matter as there are mechanics built into the game to make sure the game emphasizes and uses the connections the players have chosen), and character creation is designed to flow quickly. With that said, 13th Age still has eight races (plus three optional), eight classes and 450+ feats with which you build your character (most feats though are tied to particular classes, so there is good focus, and most modify basic abilities rather than stand on their own) and that makes for enough variation and customization for our group.
In combat, players are engaged, nearby or far. There is no battlemat, 5' squares, or counting movement/ranges. The Escalation die replaces a lot of the detailed combat maneuvers players would do in other d20 games, by simulating the improved tactics and effectiveness of the party by giving the party an extra plus one to hit every round after the first (up to a +6) in lieu of detailed flanking and situational bonuses. Spells and abilities target single targets or nearby groups of 1 to X. In our games, all the players felt their classes were effective in combat, and we did not have any issues with one class dominating over the others that some players may have experienced with other games.
The game has some other interesting features and suggests variations to better tailor the game to group playing preferences. There are no experience points -- players level up based on accomplishing story arcs (usually every four play sessions/long rests), and receive partial level-ups in between level advancements (called incremental leveling). There are variations to help make the game more gritty/real (lingering injuries, slower healing, etc.), and options to make it even quicker in play to allow more focus on story (using averages to replace rolling dice for example). The authors have notes in text boxes in various places, where they recommend other ways to do things in the game and occasionally disagree on the best way to do things, leaving it to the group to decide how they want an option to work if they decide to bring it into the game.
Another good thing is that the one book includes everything you need to play except dice -- player manual. monster manual, DM's guide, and a gazetteer on the default world of the Dragon Empire and the 13 icons. So, if you decide to give it a whirl your entry costs are lower than with a lot of other games.
My group was playing both DND/Pathfinder and FATE/Dresden FRPG, and in my players' opinion 13th Age combines the best features of the two is such a way that we're switching to this instead because they prefer it. We like more open-ended, freeform play, and are very comfortable with having the players and DM collaborate on effects and stunts. For us, 13th Age has made it easier to play the game focusing on story/narrative, while still running on a solid chassis of rules and clever character backgrounds that matter in the world.