I'm thinking of a seer/gunslinger, and I'm wondering if I could put ebb and flow to good use, or if blaster fighters generally end to quickly for it to build up and become useful?

Given that the maximum range for Ebb is short range and even to do this costs 2 pips and activating it - depending on how you rule it as its rules are a bit inconsistent at the moment and we are waiting for an FAQ on it , as it may/may not also affect your allies unless you spend more pips on it - I would say its next to useless, most of the time it will be too expensive or not possible to use at range, although a 20 xp drop for the basic power plus ,strain strength upgrade, does allow you to use flow to heal significant strain,. Upshot Id say if you plan on it for ebb, no, for flow yes.

depends totally on what you want to achiev with the power,

strainhealing and adding successes to your next check? - well go for it.

strain AOE damage to your enemys... well not the situation a gunslinger really wants to be in...

but if it is the first this could become quite te emergency resolution when ever your gm putts you in a small room full of enemys (don't know your GM, but this is a situation my SCs are ending up in with quite a frequency... even if I don't plan for it...)