As far as I understand it (and I'm no expert), it is only mag 1 protection from holds which means you will still be held by any mob with a mag 2 or above hold who is +1 to your level. That's a lot of mobs still.
It all helps I guess but I never found it really stopped me being held. Plus it doesn't protect from stun or sleep so malta and the like are still a pain. I took leaping for the immob and knockback protection. Constantly being put on your bum is almost as much of a pain as being held sometimes. 
It was always slots that were scare, not powers for me so I had no problem taking three powers that didn't need slotting.
the hold defence is pretty low, true but the knockback protection is 100% which is always useful.
I'd only do it if you were full human otherwise you'll to re-toggle after switching forms.
My Human/Dwarf form's at 45 now. He doesn't have Acrobatics (getting SJ at 47 and Acro at 49) and by far the worst headache for him has been malta sappers. IS has far too long an activation time to hold them before they get off a short burst with that damn End Drain/Hold ray, and if they hit him with it once, he's dead... Chain held without endurance till they chip away enough HP to kill him.
As a PB you can't get a 100% Knockdown ability with a fast activation (no Air Superiority) or TP foe, so dealing with sappers is probably harder for you as an AT than anyone else in the game.
Really only have a few choices:
+ Pop Light Form (for dealing with one even level minion, that's game balance for you ><
+ Use Dwarfie Knockdown attacks (maybe pop Seekers first) and hope your End holds out till you kill him.
+ Pop a Break free for Hold resistance and then IS the [censored] out of him.
+ Use Quant. Flight to absorb his alpha and try to time it so you can IS him before he gets another shot off.
+ Acrobatics (assuming the hold mag inflicted by his ray isn't more powerful than it can handle) and IS him.
At present I tend to try popping Seekers (if it's up) and then fly in with Quantum Flight, use Build Up, drop Quant Flight, RS --> IS. Usually unless RS causes Knockback I'll still get held, since he gets a shot off before the massive-activation time of IS is over.. but more often than not IS will still hit and finishes him off, leaving me held for a few more seconds with a little under 50% endurance.
Another option is that once every 25 minutes I have the option of using Geas-the Kind-Ones accolade plus Dwarfie's hold resistance to take down a sapper in safety.
All that for a single white minion.
Did I mention I HATE sappers? 
I really, REALLY think that either Human form should get some form of mezzle resistance, or Dwarfie should get Resistance to End Drain and -Recharge. Relying on the often-not-picked Quantum Flight, or the rarely-up "Light form" to take out one white minion is ridiculous.
PB's probably get more ways of dealing with sappers than most AT, it's just we don't have one 'sure fire' way like a ranged hold or debuff or snipe. We have two attacks in RS and Solar Flare that can keep them off their feet before using IS. We have QF and Lightform. You can use the knockback in Nova attacks to take them out from range. Dwarf has three attacks that cause knockdown and once down you can very quickly switch back to human to use IS before they get up. I actually hate the stun grenades Malta use far more.
Thanks for the feedback from all.
It sounds like the mezz protection from Acro is pretty much useless (which is in line with how I found it on my defender), but the KB protection from it, and Immob protection from CJ, are more worthwhile.
As CJ's a single pick I'll give that a go, and see if the PB version of Hover's usuable for KB protection if slotted up a bit.
I'll start grooming a few pet controllers too... 
I took acrobatics on mine made a huge difference and important if you spend time in human form, less important if you 3-form.
thePhilosopher Martial Arts/Regen/Fire Scrapper
theRegulator Empathy/Energy/Soul Defender
Total Inertia Ice Blast/Kinetics/Psy Corruptor
Total Inferno Ice Blast/Thermal/Leviathan Corruptor
All I know is that you get resistance to immobilisation and knockback, apart from that, it's useless.
What Philosopher said, if you go all human form, acrobatics will definately help.
Hmmmm, my initial plans (before intentions got clouded by annoying facts) had been to go triform, although now I'm in the low 30s I'm starting to find Nova a bit too weak to be useful. Nowhere near as effective as a real blaster at the same level, and as defenceless and fragile as a very fragile, defenceless thing.
So that might get respecced out at some point.
If I go human/Dwarf biform, CJ might be acceptable as a toggle - I generally only go Dwarf if the team either needs a tank or things have suddenly turned heavy, and either way once in Dwarf I'm liable to stay that way until things have calmed down a lot. After which reverting to human without CJ toggled on should be OK.
Level 32's got the nuke power pencilled in, so might take CJ at 35...
I play a human form pb (although i have dwarf form for the occasions you mentioned and nova form for after build up and when theres a tank on the team)
I have hurdle, health, stamina and cj and I have set up a bind so that I switch between aj and energy flight with one button, and also when I de-toggle any forms, cj comes on automatically.
I can provide the binds if you like... but they're quite easy to work out.
I'm really just confirming that your plan is valid because it's how I play... I don;t have acro though and probably wont take it because it's a pain to retoggle unlike cj.
What the subject line says, basically.
) and been a bit disappointed, I was wondering what other people have found here.
My PB's just dinged 30, and has been meeting more and more mezzing villain types over the last few levels; experience with my blaster main suggests it only gets worse from here on...
Other than teaming with controllers or spending the entire mission in Dwarf the only long term counter to this looks like the CJ/Acrobatics combo, but that's 3 power picks taken up and that's a rather major commitment.
Having tried CJ/Acro with my Emp defender (we wasn't a melee fiend, and still found himself repeatedly mezzed through Acro
Do you think the Leaping stuff gives good enough protection against mezz to sacrifice 3 PB powers on?