yup, very viable even.. though i wouldnt recommend it to the beginning PvPer. By which i mean, if you have to ask this question, you'll prolly have a hard time playing one in PvP 
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As FF you wont have any debuffs and that will be a VERY major disadvantage in PvP compared to other secondaries. The trick with FF is in duels you have to abuse your repel/knockback to corner em (as to compensate for the lack of hard control in ill). In team pvp you'll probably end up as the buffbot really. My first toon was ill/ff and i PvP with him now and then. Aint the omgwtfpwnsauce as other sets, but still pretty enjoyable.
Musthave powers (and why):
Blind - your standard hold, with extra added sleep effect and decent damage.. will be your main spamming power most of the time.
Insulation/Deflection Shield - Buff up your phantasm and teammates with it, FF is about defense, so youre silly not to get this. Also insulation shield works wonders vs end drains.
Superior Invisibility - Invis with no movement penalty and able to attack from it? Yes please!
Dispersion Bubble - Defense and mezprotection vs the most common mezzes in PvP. You need this!
Phantom Army/Phantasm - I dont need to explain myself here do I? How are you gonna do damage without these?
Detention Field - While its usefulness is debatable in PvE, its godlike in PvP. Use this to disable the enemy's buffer, or that blaster that just popped aim/bu and is charging at ya. Remember BF wont break him free from phase, so this really is the ultimate disable. Of course if you only duel, you might get away with skipping this.
Spectral Terror - Fear is rarely resisted in PvP unless there is a buffer running around spamming CM/clarity/thaw. Its tricky to use this however as it is location based and quite a few people will get the timing right to evade it altogether. Still if you use your bubbles right to get em where you want em, terror is ace at keeping them there.
Force bubble - You need this to corner targets and keep em cornered so your pets can go crazy on him. Little tip, try to get fly together with FB, if you hover a slight bit above the target rather than running against him, its harder for him to escape thanks to the sphere shape.
Hasten - You need phantom army up as much as you can, hasten is critical for that.
Not "needed" but very handy to have:
Forcebolt - Especially with the changes of acro coming up, you can use this to spam KB on your target. Keeping him on the floor most of the time. Also you can slot a forcefeedback +rech in it and it has the slight (5% iirc) chance to toggledrop.
Spectral wounds - more damage, simple as that
Deceive - Confuse is tricky in PvP, but still.. its another disable. And you can slot relatively cheap purple sets in it to get some nice setbonuses.
So far the powers.. A little tip on gameplay when you PvP with ill/ff, remember for a controller you got a serious lack of hard control. You only got a single target hold you can rely on, but that's basically it. The trick I use is abusing forcebubble and/or forcebolt to try to corner my enemy somewhere, keep him there with KB/repel while I spam holds and let my pets go to school on him. Also use detention field to take someone out of the fight for a fair while, make sure you slot it 3 acc 3 rech. You dont want it to miss, and you want it ready whenever you want it.
As for a specific build, im constantly tweaking my build as it is, so cant really post one without me changing my mind again next week . Just general advice I can give is to focus on +rech and +recov, ill/ff is pretty end heavy and you want as much rech as possible to have PA up ASAP.
Well, hopefully this post has been some help, I'm sure more will post here and probably have complete opposite views than what I have. But these are just my experiences with ill/ff and havin played it since EU release. And of course one general tip for any PvP build: know yourself, know your weaknesses.. every AT has weaknesses, make sure you minimize yours. Upon that, i'll give the comment stick to the next one who'll no doubt completely demolish my story
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
That's a pretty good summary and there's not much to add except that TP foe and force bubble can be quite effective.
Octavian Vanguard
@ohmsEU and @ohms 2
Badging character: Bimble on the Union server, Badgehunter.com and City Info Tracker.
For team pvp id recomend /sonic instead of /ff since def is lol in pvp.
Acro nerf will prolly make bubblers a bit fun in pvp again tho 
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That's a pretty good summary and there's not much to add except that TP foe and force bubble can be quite effective.
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Indeed it is, however i had luck fitting most powers in already with proper slotting, wouldnt see how i could fit in tp foe in stage's build.
Which reminds me, as epic you want psi or power, psi for the extra mez protection and even more mitigation vs psi attacks. Or power mainly for powerboost. Powerboosted bubbles are ace.
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For team pvp id recomend /sonic instead of /ff since def is lol in pvp.
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I agree that generally speaking res is better than def in PvP. However i found ill/ff doesnt really suffer that weakness, because:
a) you got control to keep them from attacking you
b) with IO sets you can get a pretty nifty defense rating. On my planned build i got perma 23% def to all, even higher if i activate powerboost ofc, which i got down to 10 secs actual downtime (rech - duration).
c) PFF, being illusion primary your main skills really are "fire and forget", namely PA, spec and phantasm. If you really feel so inclined you can drop those and toggle on PFF.
Of course, its all a matter of taste in the end, and i agree that its easier to play a sonic good than it is to play a ff good, especially in team arena pvp. Doesnt mean FF will always be significantly weaker especially with the extra utility powers it has. /Sonic however DOES give you less "needed" powers imo.
And ofcourse like it has been said, FF should become alot more powerful with the acro nerf. Acro now gives mag 9 kb resist, with 2/9 being enhancable. Forcebolt has a base mag 18,7 kb.. do the math
"god, how many devs did hami have to sleep with to get ED?"
Total Cat @Stagefright
Is Illusion/Force Field viable for PvP and if so can anyone reccomend a build?
@Lost Ninja is offline today...