I think it will be absolutely great. Gintama has two big advantages over other series who tend to have a more troubled history of with movies (DBZ, One Peice, Naruto, and Bleach).
One is how Gintama's random nature prevents any problems related to timing it in a way that allows it to seem "canon". Fitting the stuff that happened in the Naruto movies in Naruto's "canon" life would be pretty hard, for example, so the whole thing rings a little fillerish. For Gintama that doesn't matter, at all. One moment Gintoki is fighting heroically with his life on the line, the next he's cross dressing as Club Hostess, the next he's in prison. That's perfectly normal for him. So it doesn't matter what the story it is, it should fit in just fine.
The other is that Sorachi always wrote Gintama into movie sized arcs anyway (as opposed to other series where the arcs are much much longer). So he's just better suited as an author to tell a complete story in that amount of time.
So I'm expecting it to be the best anime movie of the year.