My guy Dr. Charon is a demon/thermal mastermind, and I'm really happy with him. Here's what thermal offers me:
- A decent AOE heal that recharges pretty rapidly even before slotting, and has good range. With a bit of recharge and heal enhancement, I can use it to support the PC and minions or a group with little fuss, short of catastrophes.
- A single-target heal with more strength and good range. It routinely saves the bacon of a minion who drew too much individual aggro (do demons have bacon? let's not contemplate it), and when I group up, it makes a great need-something-now support for other healing and buffing.
- A pair of shields that between them cover just about everything other than toxic and psi.
- A rez, which I've not taken because, well, mostly the character solos, and minions get resummoned. But there doesn't seem to be anything wrong with it, and I presumably will take it at some higher level.
- A solid single-target mez protection, which I use on whichever demon's got Hell on Earth, or just whoever seems to be having extra trouble at the moment.
And still to come a single-target buff to damage and to-hit, a single-target debuff to damage, endurance, recovery, and regen, and an AOE hammer to defense and resistance.
I really like it. It's a great Swiss Army knife sort of secondary, covering just about everything I'd like. Give it stealth and a force field for the character and I'd be set for life. 
Seriously, though, I wasn't at all sure whether I'd like it when I started out, but I am now - with the character at level 26 and cutting a good swath through First Ward - very appreciative of everyone who recommended it.
Oh, and the heals do not require a to-hit check. Cast and go.
That seems right up my ally lol. I know the goal is to not get hurt alot but I love heals to fall back on and since im not taking many attacks I need something to do 
So buffs, debuffs, 2 heals that's hard to beat. Anyone else have a suggestion? What does sonic bring to the table? I know it has resists to stack with its own.
My first demon summoner leveled to 50 quickly and enjoyably and he has thermal as a secondary. With the buff to AoE shielding you can provide good resistance to your team, your league with two clicks. Forge is the to-hit/damage buff powers. With extra recharge and IO bonuses, you can have forge on 3 pets perma and almost perma-Hell on Earth. It's quite versatile, BUT the mastermind is subject to mezzes. It's a consideration.
Demons/sonic I started with I21, and it performs better than I expected. It's not as rounded and thoroughly rugged like dark, traps, thermal, and storm. (I love storm for positioning and Freezing Rain debuff), but it isn't a slouch. Sonic doesn't have heals, but it DOES have a big sonic dispersion for AoE mezz resistance.
Sonic disruption is a beast of a power, eats end like nothing except maybe choking cloud. It's a debuff anchor on a pet/teammate/ally. It's mobile, no cooldown, teammates have to kill the anchor indirectly 
. Demons have a lot of good candidates for sonic disruption: cold demonling, lieutenants and the Prince are all good selections, and rank of the pet makes no difference to the amount of debuff.
Thermal is more survivable. As soon as you get sonic disruption, demons/sonic is very strong on offense.
Triumphant Defenders Forever
Psylenz FF/Psi, ArticQuark Storm/Rad, Symon BarSisyphus Bots/psn, Max VanSydow Thugs/Dk, Cyclone Symon Bots/stm, Blue Loki Ice/Cd, Widow 46526
HelinCarnate:OMG it is so terrible. I have the option to take 3 more powers but no additional slots. Boo F'ing hoo.
Welcome to the fun house ... the demon fun house!!!
I love demon summoning. Its pretty awesome those little buggers run around happily doing your bidding.
I have currently the dark secondary and to say the least it isn't my cup of tea. I like debuffing but everything activates so slow and the heal leaves alot to be wanted.
What im looking for is a secondary that has possibly a single and aoe heal that doesnt need an enemy target to use. I want to buff my boys and debuffy my many foes. Ill be soloing alot and using mainly so's with some hos hopefully in the future. Is there a secondary that does all that? If not I hear sonic and thermal pairing is good with demons ... why are they so good and do they have what im looking for.
Hope to hear from all the pet experts.