Firstly, my Ultra Prime is not technically an official mode. I use the wave 1 Optimus torso, not the coming-soon Battle Core remold. I don't really care, though. I have no plans to buy the Battle Core version; I'm very happy with the configuration I've got.
Second, damn is Hasbro didn't screw up the chest-armor gimmick. How on Earth Hasbro can't manage to get a toy with not one, but two 5mm ports to not stay on in combined mode when they've been using 5mm ports forever is beyond me. I managed to fix the issue with Menasor/Blackjack, but trying the same fix with Optimus/Hot Rod didn't really work; I may have shaved a bit too much off of Optimus's posts. I'm going to try adding a bit of superglue, but damn of the whole deal isn't annoying. Kind of puts a damper on the experience. Still, Hot Rod looks good as the armor and it's a nice call-back to Battlestars (which is one reason, I'm sure, why Battle Core Prime has the new Star Convoy-styled head).
Third - I'm loving the Energon Optimus emulation. Optimus himself has enough Energon-inspired details on his own, but the limbs are almost exactly in the right colors. Only Ironhide is a bit off, being red rather than orange, but then OP-1 was originally going to be red anyway, so whatever. Subbing Alpha Bravo in for Prowl only strengthens the look.
I love that Prime gets an axe. Yes, it's only because Ironhide is a retool of Off-Road and yes, it's tiny, but still, it makes a lot of sense.
Also, the silver-painted foothandguns looks really nice. I was surprised that all four deluxes got this kind of treatment (I was pretty sure Sunstreaker's would be painted since Breakdown's is too). It really helps set this wave/combiner apart.
Placing the combined rifle in his right shoulder/hand port is a nice nod to Apex PMOP.
Optimus is a lot more stable than Motormaster. Markedly so. I like him to be paired against a Stunticon-wearing Galvatronus, so it doesn't matter much anyway, but it's worth noting.
Overall, it's pretty awesome that I have a legitimate G1 Optimus combiner!