Just wait until you're both on SOs, although even on DOs it's almost there. But on SOs, two FF bubblers == god mode for the whole team. You'll be able to snipe all the time even if you do get aggro, because they have to actually hit to interrupt you.
To address the other question, the downside is the same as for any other non-Empath. "R u a healer???" 'Nuff said. I've had ice tanks or SR scrappers (i.e., people whose defenses do stack with the FF) leave the team in protest, because they just have to have a healer. Or I've had trouble convincing people to invite a second FF defender I knew -- and you've seen what kind of pwnage that means -- because while they were gracious enough to accept that they invited one non-empath, they weren't going to do the same mistake twice.
Expect this problem to actually rise through the 20's and peak somewhere in the 30s.
Good for you.
I think Defenders have been neglected for a while as ED put a lot of "Offenders" off. Having said that, their lack of popularity has made me roll a few recently. Including a Storm/Elec and a Kin/PSI both of which are great although for me becuase I solo a lot the Kin wins.
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Bubblers just seem so hard in pairs
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This is very true. It's true for most defenders, but particularly those with single target buffs (ever seen a couple of Kinetics defenders working together? It's very nice 
).
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I've noticed that FF aren't mentioned very much on this forum. Is there some big downside to them that I've yet to encounter?
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I think in all honesty their high level power picks don't rank up there alongside things like Dark Servant, Fulcrum Shift and so on. The powers are good, but a bit more situational or take quite a bit of nouse to use in a helpful way. The other thing with FF is that it's a really passive set, and the buffbot playstyle doesn't suit all... but I agree, FF is a cool def set, especially in pairs. Mind you, just about any pair of defenders is amazing -- paired kinetics and a team of scrappers/blappers anyone?
You'll find with Forcefields that you won't want all the knockback attacks as people will just complain (unless you get into a team that'll apprecuate it) meaning that you'll probably end up with a build that Bubble's up and then blasts, but being a defender it's only really supporting fire, but seeing as you won't be using your primary your end useage won't be so bad compared to other defenders. The nice thing about you taking darkis that the inherent (and slottable) to-hit debuffs in every dark attack stacks with the defence from your bubbles, meaning that AoEs will be the way to go for maximum survivability.. If you stick with it until SOs, you'll find that Dark Pit heavily slotted for accuracy along with Tenerbarous (I can never spell that...) Tentacles will be a good combo, basically forming an AoE hold. With those two attacks plus the cone (whose names escapes me at this moment) and the snipe to pick things off, you'll actually be quite active at supporting the team. It'll just take a while to learn what to target with the snipe to get max effectiveness if you use it combat; you'll find your target may get killed quite alot in the later levels before it fires 
Have fun dude!
Night Fall
"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!"
Rolled up a bubbler for the first time yesterday. I'd decided that TA/Archery was a bit dull so a friend and I made pair of FF defenders, I took Dark secondary and he took psionic. After a day of playing them I've noticed a few things.
a) We weren't what I thought a defender would be, we were a pair of blasters with high defence.
b) Because bubbling people doesn't cause much aggro, we were able to use snipe in combat almost all the time, so long as we stayed out of melee.
c) Two identical characters both using Flurry on a mob looks very cool.
d) I absolutly love defenders.
Bubblers just seem so hard in pairs, we played all day and hardly dropped below half health once. Only time we died was when we got cocky and tried to do some high level hollows missions and even then we were the last ones to hit the floor.
I've noticed that FF aren't mentioned very much on this forum. Is there some big downside to them that I've yet to encounter?