I find quite a few of the fantasy/Cybertronian/futuristic jet modes aren't terribly aerodynamic. A lot of them would never be able to fly in atmosphere when I think about it... so... I just shut down that part of my brain and suspend disbelief.

Having said that, there is something quite cool about the more realistic jet moded TFs - particularly the MP seekers. While the issues with the robot modes are widely known, I think they're bloody fantastic in jet mode.

Even the original G1 Seekers are still pretty damn cool in that regard too.