Here is my breakdown on these:
FLAK w/OUTBACK: Upon closer inspection it looks like the FLAK is the exact same mold as the original. I can't see from a side by side look at the original and new one any difference. The paint scheme is obviously different, but the mold itself looks exactly the same. OUTBACK looks great. I would hazard a guess that he has a new head, new web gear and gloved hands, but otherwise I would bet the rest of him is reused parts. Overall a great figure and an ok "vehicle", but I can think of a lot of other smaller vehicles I would have preferred first. On the flipside, I can't really be too upset with an original 82 mold.
CLAW w/STRATOVIPER: WOW, what a difference a colour can make. The red really does make this mold look a lot better. It is still a crappy update on a great toy, but the red is striking. It is nice to see the extra bombs as well. The STRATOVIPER is one of my all time favourites, and I"m glad to see him. From what I see, he has a new head, and the vest/web gear is added on, and not molded to the body. So I would say the those are the only new things (oh and the shin guards). The rest is probably once again reused parts.
STINGER w/STINGER COMMANDER: This a a great vehicle, and I love the little updates done with it. The same differences from the VAMP are there that the original had. A new front bar, no gas cans but rather step platform and bar, a cannister instead of pack on the hood and best of all... a gun instead of the shovel detachable on the front. The STINGER COMMANDER looks to be a simple repaint of the trooper mold, much like the STINGER DRIVER, except he has the COBRA BAZOOKA SOLDIER head, red mask and web gear.
ARCTIC HISS w/ARCTIC HISS DRIVER: This one has me torn a bit. I am glad to finally see a HISS that isn't black, blue or crimson, but the all white look is a toss up for me. I like it, but I'm not sure if it wouldn't be better with a little camo. I also don't like the missile rack instead of cannons. Why would you bring a HISS all the way to somewhere arctic to only have it shoot 4 missiles and be done? I love the new rack with the blanket!! I think that is a nice little touch that adds to the vehicle. The ARCTIC HISS DRIVER is another thing I'm torn on. On the one hand, it is another HISS DRIVER, but on the other, they did go a bit extra to make him different. I think it was a good choice, but I might have been just as happy with another Snow Serpent.
So, the short of it is, I would say all 3 are must buy. The FLAK/CLAW moreso for the figures than the vehicles. The STINGER is just too iconic to pass up. The ARCTIC HISS is a nice chance for the mold and comes with a new take on the vehicle and driver so I would say it is a good purchase.