Flan, if she is fast enough, can deal some *serious* damage in a short span of time. From my own usage, I can think of her as a faster-but-fragile-er version of Nitori(who I have also used and loved ^.^).
Utsuho, I used for a while, thinking "Who cares if the formula's aren't *that* great, it's freaking Utsuho!" and she preformed rather well(then again, I say the same about Remilia). In postgame, she's got reliable enough SPD(outruning anything that isn't Aya/Chen sort of omgfast), and can usually either wipe the trash, or leave them close to dead, with either Tokamak OR Reaction. If I pull out Giga Flare? They just plain die, unless they got some wicked HP and/or Mys Affinity And if Utsuho gets hit? pffft, like that's an issue, her defenses are naturally good enough that's not an issue unless the attack was something big.
Mystia... just looking at her stats, I find it hard to believe that she is as good as people say. I suppose she's like Reisen then? "Better in practice then on paper" or such? I look at her stats, and I think "slower then Chen, but better stats overall, but still nothing special."