I agree with the other reviewers - the only fault in this movie is the length, it's simply too short. But it's too short, not in the sense that something's missing from the story; rather, it feels too short because you'd want to see more of the lives of these characters.
It's a very realistic movie, almost documentary in places, with handheld camera and a somewhat gritty picture. The acting is subtle - for instance, when one of the girls (who has a crush on Jagger) questions Yossi about his lover, thinking they are just friends, you clearly sense the mild panic he's feeling by a slight change of posture, frown, etc.
And there's the scene at the end where Yossi mouths the words of the song of which Jagger sang his very own version earlier on - I can't really describe the emotional impact of that moment.
Yossi and Jagger are gay lovers but homosexuality is not really the issue of this movie. Jagger seems ready to meet the world as the person he is, he still has a bit of a child in him (and is all the more adorable for it). Yossi is the more hesitant and his responsibilty weighs heavy on him. Consequently, there is some friction between them. But heterosexual love is shown to be just as difficult. The theme of the movie is rather the fleetingness, but also the intensity and depth, of human happiness.
In that sense I see why people compare it with Brokeback Mountain, an excellent movie in its own right. But the moments of "high drama" and "big scenes" that still show through in Brokeback are entirely absent from Yossi and Jagger making it exceptionally truthful.
The dialogue is in Hebrew so you may wish to watch the film a couple of times, paying attention to the subtitles to begin with, then later on concentrating on the picture. I noticed that as I got used to it, the sound of the language in itself seemed to convey a meaning (I know no Hebrew at all!). I certainly wouldn't want to watch an English-language remake!