It depends on the character and level. I'm currently playing the Decepticon campaign with Brawl, and sometimes the tank mode comes in handy when I just wanna pack an extra punch. I'm playing the Autobot campaign with Optimus Prime, and some of the levels where you have to race along the highways are easier done with vehicle mode (although robot mode is still possible).

I find the 'Prelude to Energon' (Armada) game was better in terms of engineering levels that forced you to use alternate between vehicle and robot mode. But I'm still pretty early in the game so I'll see how things pan out later.

Now off to rescue Zeta Prime - this surely won't be a trap!