Optimus Prime in robot mode has about as little resemblance to his movie incarnation as is possible while still being recognisable as the character. The head is the same and the chest at least tries to look like it did on the screen, but the rest is pretty different. Posabilty is okay, but nothing spectacular. All in all a robot mode that does okay, but no more than that.
Unlike the robot mode Legends Prime makes for a very nice truck. The vehicle mode looks pretty good and matches the truck seen in the movie as well as it can at this size. Absolutely no complaints about the vehicle mode.
I got the Legends version of Optimus Prime mostly for completness' sake, little more than that. It has a nice vehicle mode, but the robot mode is pretty forgettable. If you want an Optimus Prime from the Movie, there are quite a few other excellent versions out there. No need to get this one unless you got a thing for the Legends class.