Finally cracked open the SDCC Exclusive Alien Egg Chamber last night and wanted to provide a brief review, sans pictures.
It is, as I figured it would be, a very well done effort overall, not perfect, but pretty great for what it is.
The base is very heavy, moreso than any of the vintage Star Wars ones. It is similar in mold though in that it's more or less hollow, but it's dense, much more hefty than I expected. The coloring is definitely more of a darker charcoal grey and there is no paint at all applied. The eggs appear to be molded separately, but firmly stuck in place. One of them is open to simulate the scene from the movie.
The cardboard background is pretty nice, well painted, gives some illusion of depth and perspective, with a tiny rendition of the Engineer at the very top, as if at the end of a long upward arching hallway. Like the old Kenner playsets, it comes with additional cardboard "feet" you need to fold up to attach it to the slots in the base.
The only real gripe I have is that the majority of the footpegs appear to be too large for the peg holes in the figures' feet. I tried them out with both Kane and the Alien, but could only get them to fit onto 2 out of the 7. Maybe they would work better with the other characters, I haven't tried them with the ones from the Mystery Eggs yet, but it seems like those should be universal.
Speaking of which, I have been able to snag 5 of the 8 Mystery figures so far, just need the clear Alien and both clear Ripleys and I am caught up.