I'd think you wouldn't need another review with this many positive ratings, but just to be sure... I'm going to tell you to go watch this too. =P
Obviously, that is if you like a healthy dose of nutty people, time travel that has actually been thought through! A bit of romance (even sort of touching harem at some point, but can be neglected), drama for some possible teary moments and lots of laughs.
You just have to watch this.
Ofcourse, if you're going to nitpick at the concept of timetravel, you will always end up at the impossibility and the approach on this anime (which I have seen before in Hitman Reborn, but not executed as well) may raise a couple of other questions, but all in all, the whole story is very well put together and it doesn't feel like they paradoxed themselves to death like some other series did or do.
So again, you just HAVE TO watch this anime if you haven't already. And if you have, well, you might want to save it, because you may want to watch it again.