I love combustion but everyone else around here seems to hate it. So I'll say keep it and be outvoted soon. Reasons I love it... Recharge, size of target area, nifty animation and fireworks.
"Well, they found my diary today.
They were appropriately appalled
at the discovery of the eight victims
They're now putting it all together.
Women wrapped in silk
with one leg missing
Eight legs, one body, silk,
spider, brilliant!"
If you can get better powers instead (pool or aim/bu) i would pick those first and only keep FSC. Most fire/fire wont go 'AoE style' untill the higher lvls anyway.
50)Sinergy X/(50)Mika.
(50)MaceX/(50)Encore
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Combustion is great. Adds to the already massive burn damage that the enemy are taking. All you see is orange numbers everywhere. Some combining to make really usual high numbers like 2020 which would one shot most things. Then you realise that it was just a couple of ticks from 3 of the DoTs that are on all the enemy around you before they fall to the ground and you leave the building with a fresh smell of barbeque. That is the joy of the Fire/Fire. 
I didnt respec into my AoE 'Whirling death machine' build until I hit 50, but I personally would drop Combustion, with Fireball, Fire breath, FSC you've got your AoE attack chain there
Hawk
Am looking at maybe using up one of my Freespecs on my level 18 Fire/Fire Blaster to drop Combustion.
Whilst I like the concept of this power and think it fits well in the sets "thematically", I find it does very anaemic damage currently with two damage DOs in it. I already have FSC for a decent PBAOE and am wondering is I really NEED Combustion as well?
If I do drop it it will also allow me to free up slots and get three damage slots in all bar one of my attacks at my currentl level (18)
So my question is should I be patient in slotting up my attacks and keep Combustion in because it will mature later in the build?
OR
Do I get rid of it and max out the damage early on my current attacks?
Any thoughts from veteran fire blasters?