I was going to post my reviews on Hearts of Steel Starscream and Optimus Prime, but this guy showed up and I've been fiddling with him for most of the afternoon, so I'll do him instead; I can still review the HoS guys later if people are interested.
Anyway, Star Optimus Prime (AKA the Thirteenth Prime). First, I want to point out that his chest windows were perfectly fine - no misassembly. I've had no issues with him - everything is nice and tight and locks into place as intended, with no defects to note. His legs are indeed hollow, but it's by design - they open up into segments for his base mode and serve as storage for all of his blast effects and accessories when they're not in use; it has no bearing on how solid he is in robot mode and was a thing even on the original toy, so it's a non-issue.
He comes with a slew of accessories - two small turrets that plug into his shoulders and can move up and down, his big blaster, a smaller gun that unfolds into a turret for Hot Rod to use, Hot Rod himself, a 'hilt' that can combine with blast effects to create an energy 'sword', an MT-13 Micro Trailer that makes a nice stand-in for Roller and can be turned into a turret, and a LOT of blast effects. He also includes a Zodiac Orb, as well as a blue Energon bullet that seems to serve no purpose, cannot attach to any pegs or ports on Prime nor the barrels of either gun, and is completely absent from the instruction manual. It seems it fits into a slot on the right side of Prime's big gun, but that's about it. The sword hilt and both guns can all be combined together to form a slightly longer gun, which is apparently supposed to be his 'Big Bang Cannon' from the previous (horrible looking) Generations Selects release of this guy. Incidentally, the sword hilt doubles as a Matrix holder, and is apparently designed so you can plug the Zodiac Orb on top, the Silver Matrix from the GS release on the front (in the Matrix-shaped slot), and then plug the whole thing into his chest.