vehicle mode was my justification for buying mp-04, as well as magnus (who now get's displayed in robot mode along mp-01) and I bought two mp starscreams for the same reason. if megs is any good, I'll buy another of him as well.
I wish I could afford to buy two of every masterpiece
But in any event, I agree that the robot mode is awesome, but to me, Transformers is more than just kick ass robots!
They're kick as robots that transform! Being a truck is a part of Optimus as having a faceplate, a big ass gun, ass kicking "boots" etc.
Granted, it does SOUND kind of cheesy to pay X amount of dollars to have a fancy truck displayed, but as I said, the truck mode is also a part of Optimus.
My initial dilemma wasn't so much MP-01 vs MP-04: Which is worth the money, but rather a comparison of the QUALITY of the Optimus Prime between each Masterpiece.
I fully appreciate ALL the comments made here, it really helped me out a lot!