I have the problem of being undecided. I love being the Engineer, but the psychic abilities are just unbelievably awesome. Admittedly, I found my 'fix' in being able to go Engineer, with 'Push' as an ability.
Engineers are good in that if you can give them a secondary weapon, like...Assault Rifle, or (my preference) Shotguns, then they are indeed quite powerful as a main-character. You can both heal and protect your team mates while at the same time dishing out some major damage with your Engie-mines and your main weapon. I tended to have Tali along for her Shotgun prowess (and her shields...she just wouldn't die), and I had Wrex coming along so he could run up and beat the snot out of whatever looked at him funny.
I did take the 'Agent' sub-class for the more offensive abilities though, admittedly. I was tempted to go with Infiltrator, but...I liked having that extra heal buff.