I originally posted a form of these thoughts on Bolter & Chainsword in the New Codex: Space Marines, It's going to be huge thread.
Basically, there's a persistent rumor that Storm Shields are going to confer a 3+ Invulnerable Save (sometimes written as "3++"). Supposedly, this rumor has been confirmed as true from multiple sources, but I'm still a little skeptical.
Regardless, I wanted to interject these thoughts:
OK, so I have a box of assault termies and I was SO looking forward to giving them all lightning claws, joining a TDA chaplain to them and on the charge playing "suck on this". Looks like NOW, though, there's an important decision to be made...do I want a CC wrecking crew that can shred pretty much any unit I need it to (can you say Point. Click. Die.?) OR do I want to mix-and-match in some SS/TH termies OR do I want to build a very slow (I1...*sigh*) but VERY durable SS/TH-only squad?
I think it's time for some magnetized arms, myself.
Oh...and while I'm all about a 2+/3++ configuration for termies myself (I think they should all be that way, if they're going to come even somewhat close to matching the fluff) I do want to point something out (even though it's going to sound like I'm parade-raining): that damnable single pip, AKA the 1, has a nasty habit of popping up when you have to roll a handful of dice in response to, oh, say, massed gunfire or Khorne Berzerkers in CC. As sexy as 2+/3++ sounds, Tau/IG/Bladestorm-ed Dire Avenger gunlines will force enough saves for a 1 or 2 to pop up more often than you'd like (which is NEVER, am I right? :) ) and if there's only 5 in a unit...well, even one 1 is one too many. Same goes for Harlequins/Berzerkers/Chosen with a pile of PFs. I know I'd love to think 3++ is great -- oh, and it IS, make no mistake -- but it's not invulnerability. Remember this: every time you have pulled a marine off the board and put him back in the tray -- if you even got to roll beforehand -- you rolled a 1 or a 2.
Sorry for the sobering thoughts...but we're partying like we've been passed out 1+/1++ saves...and we haven't.
Personally, while I hate, hate, hate "the fluff argument" (even though I somewhat ventured into this territory above) I think termies should be 2+/3++, T5 with FNP...in accordance with the fluff. They would then cost 75 points, but I'd pay it, and it might even make me stop preaching that holofields should cost 1,000 points (which they should -- I'm looking at you, tri-Falcon list).
The point I was trying to make is that while 2+/3++ SOUNDS really, really good, it ISN'T invulnerability. It's a damn sight better than 2+/5++, and it will definitely come in handy for those times when you REALLY need an invulnerable save, but without some kind of re-roll-ability (which, come to think of it, with the new FNP rules, wouldn't help for anything you'd need to roll an invulnerable save against anyway) it's really not all that great.
Consider this: Tactical Dreadnought Armor is supposed to be impervious to small-arms fire. However, more often than not, it's massed small-arms fire that usually kills my termies. If you have to make a pile of save rolls, I can promise you that you're going to get a 1, and there you go: dead terminator, at the hands of small-arms fire. I know that game balance demands this, but it doesn't mean I have to like it.
While we're on the subject, I'd like all penetrating hits against a Land Raider to be downgraded to glances, a BS10 Vindicator with 60" range, and a rending pony. :)
