Well, if you were hoping for a big explosion graphic... yeah, not impressive.
Like all AoE effects, it's pointless on single mobs. However, that's not to say it doesnt have it's uses.
Fire Nova has a
100 yard range. It's at least something you can press when you're moving, or when you're at least outside the 40 yd casting range.
Ordayc, on Wowhead, Fire Nova comment wrote:
With
Reverberation and
Glyph of Flame Shock it is possible to trigger Fire Nova on four targets. Each Fire Nova deals damage independently, which means a fifth target could suffer damage from all four Novas.
Also noteworthy is that its range has been increased to 100 yards (110 with
Elemental Reach), which means you can trigger it over huge distances once you applied Flame Shock, e.g. against people running/riding away in battlegrounds (however, since the runner does not take damage, this is of limited use).
As mentioned in the quote, this does have some situational uses in PvP, particularly due to it's range.
I can confirm that the multiple flame shocks/novas aspect does work, from experience, although at most I have 2 targets lit up.