In addition to the previous reviews, which effectively describe and summarise this film, here are a few comments to help you decide.
Firstly, this is an excellent film, but it is no all-action surf movie - so don't even think about buying it if you want to see tsunami-sized breakers and tunnel-shots! The surf gets to about three feet (at most) and the beaches are mostly black ash: an interesting contrast to the more familiar North Cornwall coast! It is also fascinating to see the Japanese take on surf culture - it is recognisable, but very different at the same time. In my opinion, you'll get more from this film if you have an awareness of Japanese culture and enjoy films of a "world cinema" genre. There are things you might like to be prepared for and accept as part of the much bigger picture, some of which aren't exclusive to Japanese films, of course.
Typically, the visual aspect is often more important than the narrative: there is very little dialogue and, when the film concentrates on the two main characters (both of whom are deaf), there's no sound. You experience what the characters experience, although I didn't notice this at first. You might find that some of the acting seems to be improvised or un-rehearsed; some dialogue might appear to be unnatural (especially the comments of the surfers about the main character, Shigeru, as he attempts to teach himself to surf with a broken board). I think, though, that this is because most films are over-rehearsed and over-directed in comparison (or at least closely controlled). Life is, on the whole, pretty slow-paced (dull, even) compared to the action films we are used to watching.
There is plenty of comedy, a lot of which is very subtle. I'm no Japanese expert, but I know that there are things I laughed out loud at that would go over many heads. (I have the opposite problem with Shakespeare - I can't understand it at all, yet those around me seemed to think it hilarious!) I especially enjoyed the antics of the two "friends" who began surfing when they started to think that they were missing out on something, and made more of a mess of learning than Shigeru did - wetsuit put on backwards and tripping over the board leash (which makes Shigeru laugh in the same way that others laughed at him) and they even turned up to a competition they'd not entered. Shigeru's acceptance into the group is touching, even though he is still clearly out of place, and his eventual success is inevitable. His totally dedicated girlfriend Takako stays with him despite hardly ever even being acknowledged. There are parallels of a stereotypical, old-fashioned Japanese role-model of the wife that you see in Ozu films; she picks up and organises clothes for him, walks behind him when they're out "together" and sits waiting patiently for him to return.
And just as things are all coming together nicely, the ending brings a totally unexpected twist ... but I'll not spoil that for you! You'll just have to buy it and see.