Curtain Call is worth playing through, but definitely pales in comparison to the original. For starters, it has only one route; what few decisions you make in game have absolutely no effect on the plot outside of changing one or two lines of dialogue. There is only 1 (decent quality) H-scene. Also, there are far fewer original songs for the game, and by and large they aren't as good as the songs from the original.
As was mentioned, the new main character is a talentless loudmouth whose only charm is his sheer bravado, although he actually grew on me and I ended up liking him. Otherwise the characters are generally less interesting than the characters from the first game, and your inability to influence the story makes it so that you can't explore the characters that you do find interesting further. There are *some* plot twists though, although none even come close to the plot twists in the original.
Like the frickin horribly cruel one in the first Kirari route, for instance.
It was generally well written enough to keep me interested for as long as it took to play through it. If I could travel back in time I wouldn't need to warn myself to not play it, but if this had been the original I wouldn't have been dying to play the sequel.
In short, Kira Kira Curtain Call is to Kira Kira as Edelweiss Eiden Fantasia is to Edelweiss, except that Curtain Call has *one* route whereas EEF had two, even though EEF was marketed as a fandisk and KKCC was marketed as a full game. Although, yes, Curtain Call's one route is a lot longer than EEF's two. Still not a particularly worthy successor, however.
And that's basically the size of it.