This is a bad boy. It does come at a cost though. I just got this at level 40. The main cost is that now my corruption has a casting time (4 points in affliction will instant cast it). But for this spell it is worth it. With the new patch soon to be released they are going to extend the range on it, currently you can cast immolate from farther then this.

The main boon to this spell is how you can 1-2 punch them so fast. Your immolate already crits pretty often and at level 40 it is not rare to see it hit for 360 or so. (that pisses off creatures but great for PVP)
Next hit them with one more dot, a searing pain or what have you (again great crits fast cast time when all in destruction) then hit them with this. It casts fast enough and will crit often. Not rare to see 420's with a crit at level 40. Wear a robe for added fire damage etc. You can see how quickly you can get someone down. In fact I can see shadow bolt becoming a thing of the past due to its LONG.....SLOW cast time. It becomes faster to immolate, then conflagerate as opposed to 1 shadow bolt.

That is of course if you don't care about your mana usage.

If you are going destruction then this is a must have at level 40. Do what it takes because this is awesome. I know little about the other tree's though.