FFT is my favorite game of all time, so you could say I highly recommend it. That said I can easily see how it wouldn't be someone's cup of tea, but I think if you are a fan of the Final Fantasy series you absolutely need to give it a try.
I could talk all day about how awesome it is, but it would be better if you just played it.
JP are doled out according to this equation:
JP = [(8 + (JobLevel * 2) + [Lv / 4]) * M]
where JobLevel is caster's level at their current job
(can range from 1 - 8)
Lv is the level of the person performing the action
M is 3/2 if the caster has 'Gained JP-UP'
1 otherwise
JP SHARING
When unit X gains JP, everyone else in the party will earn
[gained JP / 4] towards the job which unit X currently has.
Note: if unit X has 'Gained JP-UP', the extra JP given by that
ability are ignored in the JP sharing equation.