Infinite is well done, a clever concept, and generally solid, but unless you're a huge Disgaea fan, it can be tedious.
It is a pure adventure game -- there are no RPG elements to it, no combat, etc. It's dialog-driven, and involves you replaying the storyline as many, many different characters to solve the game's mystery. You can bounce between characters at virtually any time, affect dialog choices characters make, etc. as the protagonist's "spirit" bounces back and forth through time. It's a cool mechanic.
The game has a lot of humor throughout, and the production values are terrific. My issue is that though you can fast-forward through sections you've already seen, it's still a very tedious process when you want to take another branch late in the story, hear the same jokes for the 12th time, miss a cue, etc. It just started to grind on me after about 5 hours. I get the feeling that bigger fans of the Disgaea universe would love all the nods to the games characters, sub-plots, etc. but it wasn't enough for me.
The good:
* All dialog is spoken, and very well voice acted
* The visuals are wonderful -- it's like playing a series of cut scenes because of the high visual quality. Occasionlly the story will jump to show a bit of action using the traditional smaller sprites, and it's a really fun effect to show more cartoonish moments in the traditional game world look.
* Lots of humor throughout
* Very creative time-travel gameplay model
The not-so-great:
* Very repetitious play (though for the price, it's forgivable)
* Chock-full of inside jokes for the series, so the humor can be lost on you if you don't play that series' other games.
Overall, for fans of Disgaea, it's well worth the $15-$20 you'll pay. For people new to Disgaea and just in it for the adventure aspect, rent it.