Everyone pretty much told the nitty gritty already regarding the "math" of FlS, so I won't beat the dead horse.
In practice (pre-2.3, course I'm sure this application would just be "better" in 2.3) as a Enh, I tended to opening a fight (I'm talking partys and raids, not soloing generally) with FlS first, for a few reasons. Firstly, to help "control" hate spikes from melee crits/WF's I usually had to start a encounter a few seconds behind all the other dps, which typically is our only "hate management" value enh has for the most part. FlS allowed me to "get on the board" for dps, without the fear of spiking it, giving me more "free" dmg even though I haven't completely committed to full melee yet, not to mention saving my burst dmg combo (SS+ES) for when I felt that hate was established on the MT. Plus, FlS helped keep steady DoT dmg on top of melee dmg, edging overall dps up just a tad. Problem was pre-2.3 that using shocks liberally ment I ran outta MP very fast, so FlS tended to be ONLY a opener, course thanks to SF we can spam shocks freely now.
I do remember when I was a Resto, FlS was bascially the MAIN source of "dmg" when I was soloing, especially if I was taking down a elite(70+). Since a typical "pull" only allowed for about 2 LB's before the mob was in your face, shocks we're key since they we're uninteruptable, compared to LB's or CL. Between slower cast times when casting HW or LB's, the time is better spent "healing" rather than trying to squeeze off a LB/CL, when taking hits for slower casting. So with FlS up (and Searing tot), it allowed me to keep some dps, while I was squeezing out heals.
As a Elem, I tended not to rely on FlS for dmg (while soloing mostly), since I built much of my setup around crits and LB's. So frankly, for the sake of keeping things time efficiant, I usually killed something with LB's before the the duration of FlS's DoT would have been up anyhow, so FlS would have been a moot point. Granted in longer fights (raids) I tended not to use FlS, just simply because I forgot too, and my mental MO was more focused on spamming LB's. I'm sure it would have done its job well, if not better than it did when I was a full resto, I just tended to mash my LB macro faster than my shocks :P