The Detonator is fun but it's not very practical. The explosion radius has to be incredibly well timed, and judgement is hard to use with it, due to travel speed, angle of view and so forth.
The flare jump isn't even enough to get on the 2Fort courtyard stairs.
It doesn't crit when it hits burning players, and if you're using the detonator I don't think you should be trying to get a direct hit anyway, that's the flaregun's job.
Best I've ever set on fire was 3 people with one flare.
Two situations I've found it useful for were; When a Medic was behind a Heavy and no one could get a shot on the Medic. Shot over the Heavy's shoulder and detonated it, hit both of them. Three detonates later, Medic died before an uber was made.
And hitting people who are around corners, but... They have to be pretty close to the corner.