I got a L36 Grav/Storm and he is great fun. Wormholing a mob to a corner, turn on Hurricane and spam Crushing Field and Freezing Rain is a beauty to see. Sometimes it goes wrong, but most of the times I get compliments from my teamates saying how cool is my playstyle and how awesome is the control potential of a Grav/Storm.
Plus, Singy is my favourite pet. It got awesome accuracy and resistance and helps spaming holds, immobs and lift. Also, it got a Repel power (similar to Kinetics) which is great to help keeping the foes cornered or away from melee range.
I'd also recommend to consider a Grav/TA. Sure, it blooms very late, but it's great fun. Debuffs + Wormhole + Oil Slick is a blast.
Take Magic or Tech origin so you can have an ignite power for Oil Slick. Oh, and you'll be wanted for Hami Raids for mass rez 
@False Fiction - Virtue / Defiant
Current projects - [Glaciologist - Ill/Cold Troller] [Cloudshaper - Storm/Dark Def] [Harald Wartooth - Elec/Psi Domi]
Go for Radiation. So it has rez factor if you take the mutation. Old Psy-knight ( i missed him 
) was a Mind/Rad controller but i never include Mutation into build. Okay it was in build but as i have whole set of Mind tricks i kick it out.
You are a controller and controllers don't have to be defenders. If a troller has rez it is a nice thing for team, if not heh there are always wakies, hospital or temp self rez from bank mission. Hmmm i just remember i didn't have mutation on my radiation defender too.
Other set i love from the defeneder AT is Force Field but you point your situation out. Grav/FF always remind me of the Magneto. ^^
I have a Grav/FF
If I were to do it again, she'd be Grav/Kin or Grav/Storm, probably the latter. Take from that what you will.
Grav/TA is a super combo with lots of synergy. (@False_fiction : Wow, another Grav/TA! Cool!)
If I wasn't allowed be TA I'd go Storm too, I've played storm to 50 and Wormhole, Crushing field, Freezing Rain and Hurricane is just an evil sounding combo.
Storm is earlier to develop than TA as well, Grav / TA is a bit of a slog to 26/28 and a blast from then on (just got EMP Arrow myself, probably the finest AOE hold in the game), Grav/Storm would start peaking from 18 onwards really (but tend to lull at 35-40ish).
Grav/Rad is good
Grav/FF is great
Grav/Kin in superb
So I'd say stick with the Grav/FF if you're not inclined to try the other 2.
@Jay Leon Hart
Kerensky: this has nothing to do with underwear
Zwillinger: I put on my robe and wizard hat...
Synapse: I had to resist starting my last post off with "Yo dawg!"
I'll join the Gravity/Storm love-in too. It's a combination that gives you a ton of offensive options and keeps the big SMASH of your Propel doing maximum damage. However, you may have to be a little more in-yer-face with the enemies to make the most use of it, so you'll need courage and care.
Played correctly, you're not just locking down whole mobs, you're mashing them.
If you wanted a bit more of a team support role, Rad or Kin also have good synergies.
Is it time for the dance of joy yet?
Ok, so typically after trying all the other ATs, I pretty much love trollers, I have a 34 ill/Kin that I leveled up very fast, about 2 weeks.
Now I have decided to try Gravity, because it seems a bit quircky and weird, and thats me, so why not? Now the toruble is, put it with what?
ATM I have Grav/FF, but I already have a FF Defnder, who, tho I did love was too much hassle, and I was planning on rebuilding as a Troller /FF, But, even tho Grav Synergises well, in appearance and originstory wise (atleast for me) I'm not sure if I should scrap this toon, build the /FF that I want using different primary at another point, and re-roll my grav/ ??
Now the options are pretty limited, I have the /Kin and a Kin/Elec defender, so Kin is out.
FF is already on, and thats whats in question
I have a natural aversion to playing Rad, tho I enjoy the debuffing it gives, and I thank god for AM pre-stamina, I just can't get onto liking Rad :S tho it does have the rez factor.
Emp >< noway, simle enough
Sonic >< I'll tkae alot of convincing onto this, so please feel free to try!
TA >< not a fan of archery, i have a TA/A milling around, and /A is the reason my FF defnder got boring to me.
Storm, I seem to like this, plus Lightning Storm provides a second pet for when I come to trying to stealth around places and for added defence to me and my team, but does it follow u? can it be TPd? or is it immobile and unnafectable?
So really to me the debate is 1) Scrap, and get Grav/Storm, or keep at Grav/FF
Any input is much appreciated!