A 40% slow effect is noticeable, but not massively so. What were you fighting? The effect is diminished vs. higher level spawns. Some enemies are resistant to slows as well.
Characters with taunt effects will stop most enemies from trying to run and of course AoE immobs are helpful.
Rain of Fire can be extremely potent with the proper combination. A solo Fire/Nrg does not have much synergy for Rain of Fire (Power Boost may help the slow effect, but likely not enough to truly shine). APPs can change that (especially the Villain versions).
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Build Up+Aim+RoF+Fireball+Breath of Fire = Win (Unless you don't have anyone to hold the aggro in which case it usually = Debt) 
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It's all about how you use RoF.
If you Aim > BU > RoF The mobs will scatter before they are defeated or spread out far enough to limit the effectiveness of your other AoEs.
The trick is to Aim > BU > Fire Breath > Fire Ball and THEN RoF where you are standing. The mobs will be trying to close to melee range to smack you. The slow effect means they'll get one shot at you and then the Avoid + Damage effect will have them turning tail and running but by then it will be too late and they will have spent most of the duration of the RoF in its area of effect.
To answer your other question: 5 slot Posi Blast and a HO micro. Some players consider this an exploit and the Devs, both old guard and current crop, have been known to go both ways on whether its an exploit or whether it is working as (un)intended.
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Definitely varies according to enemy type
First there's the fear which makes them run out of it in the time it takes to cast
And some, like warwolves, treat it like a skating rink,... feels like it propels them towards me
Thanks for the info guys/gals, much appreciated.
So far, I haven't seen any slow effect against anyone he's fought (CoT,
Hellions, Skulls, Arachnos, etc.).
However, it occurs to me, after mulling it over and reading responses, it may
be level-based perception. Right now, he's a lowbie (L24? or so), so perhaps,
it is a simple case of "a percentage of a low number might be less obvious
than the same percentage of a higher number).
ie. say movement speed is 10 at L20 - the 40% brings it to 6...
say movement speed is 30 at L30 - the 40% brings it to 18.
It might simply be harder to see (now) the difference between 6 & 10
vs. 18 & 30 a few levels down the road.
We'll see.
As Milady's_Knight mentions, the most effective use I've found for it so
far is to cast it around me. That has been helpful - slowing them? Not so much.
As for slots, my current plan calls for only 3. A bunch of my other powers
need 6 slots (for TStrike, Gaussian, and Mako) to cap ranged def - that
doesn't leave too many for RoF. As mentioned in the first note, the 3rd
slot will (eventually) get a Ragnarok KD Proc in it which should help it's
mitigation and scatter.
Thanks again, for your respones.
Regards,
4
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there's a buyer who leaves the market dirty stinkin' IOed. - Obitus.
No, Victoria, this isn't about Incarnate / Reactive
I just recently got this on my L20 Fire/Nrg Blaster, and I'm having a *wth* moment?
In reading the description and detailed info, I see a -40% slow on mobs listed.
Unfortunately, when I cast it, I don't see any slow effect at all in actuality.
Mobs run in and out of it with complete impunity as far as I can see (apart
from that impressive 1 dmg / tick they're taking while in it).
Further, you can't actually improve the "slow" attritube since the only sets
it will take are TgtAoE (which don't have any slow enhancements).
So, what gives?
Is this just an error in the description, is it bugged, or am I missing something?
I'm planning to put a %Chance for Knockdown Ragnarok in it eventually,
but I'm starting to wonder if I should have skipped it and taken Fire Breath
instead (despite not liking cones particularly).
RoF is graphically appealing, and suits the character's concept better, but
I'm extremely underwhelmed with its apparent effectiveness so far.
Thoughts?
Regards,
4
I've been rich, and I've been poor. Rich is definitely better.
Light is faster than sound - that's why some people look smart until they speak.
For every seller who leaves the market dirty stinkin' rich,
there's a buyer who leaves the market dirty stinkin' IOed. - Obitus.