I've managed to test out everyone, though my least used character is probably Gust and Rom/Ram. Of course, Gear is too overpowered, especially equipped with that item that lets you break damage limit...
I'm pretty disappointed at the original CPU's to be honest, each only have 3~4 skills and most of them are buffs and their offensive skills aren't too great either other than Verts.
Neptune - that bit sword... which pretty much only hits twice in a small range, requires fair of SP/AP.
Noir - pretty much same as Neptune but only hits once. Requires a fair amount of SP/AP
Blanc - single target but hits like 6 times IIRC. Extremely weak. Only a bit a SP/AP compared to the two above.
Verts - the spear with the line AoE. Less SP/AP than Noir and Neptunes IIRC and much easier to hit multiple targets with and does max damage.
I've heard about the DLC content where they get skills but whats the point? By the time they come out, I wouldn't be surprised if I'm done with the game (and busy playing Tales of Grace instead).
Though my main party consist of Gear, Uni, Noir and 5pb. I'm not using them based on stats or what not, rather more based on favorite characters. I find 5pb is extremely weak end game compared to the rest the characters and the having not a single extra outfit available is saddening but I still use her just to hear her say "tuturuu~