Startin' at the top my friend -- positively gorgeous! It looks so well-done for cardboard. I'm glad you've mastered the cardboard technique so the paper material didn't ripple.
The head-to-torso proportions are very nice, and the shoulders seem to vastly help out his width. Nice building out, and I love hoe you attached the shoulders to the torso -- that seems to be the best way in making one look boxy/large!
As for the helmet itself: looking at your work log, it is, I daresay, such beautiful construction. The tape left a funny pattern under (I'm assuming) the spray painted blue, and it would possibly be best to smooth that out with a different material ... though, what what's to be, I've no clue. I know that these people
have made a fiberglass + cardboard costume .... Maybe once you're a little more confident in your abilities you could try using those techniques to smooth the surfaces? :)
Laserbeak in your chest compartment is a rather nice touch. He looks cute in there -- however, I'm a touch sad that the Decepticon symbol isn't as prominent as it should be. Against Laserbeak, it is rather dark ... I would have rather seen it as a lighter/brighter purple so it was more of a "HEY, Imma 'Con, Grr." rather than "Can has your energon, plz?"
The yellow around the tape deck rather further accentuates my previously-mentioned statement. The yellow is so bright against the dark window that it draws a lot of attention to your chest, an rather doesn't allow the eye to wander much else as it squints to see what's in the window. Personally, I also would have made the yellow darker to match the darker color job you seem to have chosen for Soundwave here (From the pictures at WalMart, the suit's blue looks rather dark). Maybe creating a solid color hue before beginning the final product might work for you?
I'm a sucker for mid-sections, really. :P So I was kind of saddened to see that Soundwave here doesn't have much of a midsection. I think it's a combination of the torso being too long/cod being too high up ....Or maybe just the torso. So, possibly making the torso a little shorter next time to make the character look a touch taller?
The hands -- don't have much to say here. I like the articulated pieces there! Also, the hands and forearms look pretty spot-on proportions. :)
The cod itself is very pretty. Boxy and there's quite a bit of negative space in it .... but it is rather well done. I like the fact that you've had it all one connecting piece. I wouldn't have thought to do that with my own costumes! Does it help hold up the piece in its place, or is it strictly in order to hold the cod straight?
Goin' with the legs, they look very cool -- however, I'm a touch put off with how the upper legs sat on the lower legs like that. 'Course, I'm used to seeing upper legs going into the lower legs so they sit inside and stay where they're supposed to, instead of one becoming skewed a little to the side. However, after looking at where the feet set in the lower legs, I've come to see that this way worked out better for you and where your feet are sitting. I still feel that the lower legs should have fit inside the upper legs, at least a little.
The feet -- killer that you put sandals in there to keep it stable! I've seen a lot of people do that with their peds and I feel that it's a really useful technique, rather than trying to build something to house your feet.
All in all, I love your costume. It's very well-made and, while looks like it could need some repairs within no-time, it's very pleasing to look at. Smashing design, awesome craftsmanship, and such a striking, familiar face. Well done!