The instructions don't show this, but you're meant to get the posts on Scavenger and hook them into Mixmaster's rear wheels before pushing the C-clips on Mixmaster down onto Scavenger. This will give Mixmaster a firm connection onto Scavenger.
I was planning on doing this once I got all eight Constructicons, but here are the individual combined modes as standalone pieces:
On Devastator himself: he turned out a fair big bigger than I was expecting, since it seemed like the combined modes were 'compacted'. He's heavy, too - the individual Constructicons may only have limited mass, but put eight of them together and you end up with something quite hefty.
I know some fans have taken exception to Devastator's bipedal stance, but I think that the idea of Devastator as a hulking brute that towers over others is better served as a biped than as a quadruped. Besides, concept art as well as the original promotional image that showcased Studio Series Devastator's components depicted him as a biped.
Gripes? The connector peg on Hightower should have had a tighter joint so that it could support the combined weight of Hightower and Scrapmetal if someone wanted to raise the left arm at the elbow, and I had to tweak the ankles a bit to get Devastator to stand when ratcheting the hips forward in order to avoid a slouching, hips-forward stance. Otherwise, I'm quite happy with Devastator and think he was worth the wait.