As much as I love Optimus standing on his own as a character and individual, I really like the newer combining toys. With the exception of the ROTF one.
Armada Prime became the torso of his super-mode, so the capability to combine with Jetfire (as legs) was a welcome one that added play value, I think. The two toys are great on their own, and the "flight mode" is a cool extra feature.
Energon's theme was combining, so Prime had to combine this time. Together with Wing Saber, he had the cool attack mode/flight mode thing going on. The toys (heck, the whole line of toys) were a little clunky, but good fun.
Now, Cybertron Prime. Woo... where do I start? With Wing Saber (or Sonic Bomber) he shares a couple features and design aesthetics with God Ginrai, and is a huge, well built, solid toy. He's great!
In short, I like the idea of combining toys... I don't necessarily want it to become a feature of Optimus Prime, though. I think G2 was the last Prime that really stood as a single robot with no other combining gimmick. The exception being Classics/Henkei - but there's been so much fuss about trailers and Magnus/Powermaster armour it seems that many were disappointed he didn't have some cool combining feature or power-up mode.