Yeah turrets are a pain, for defence they are really useful as on C&C you set them apart in such a way their range crosses over so there is no easy way for the enemy to attack wothout being attacked from both sides.
Attacking turrets or ground defences on C&C I try and use air attacks, even if there are a few AA guns nearby once you have damaged or destroyed ground based turrets (or similar) it is easier to move in with tanks, at times you just need to use units as cannon fodder so you can attack with other units. they can only ffire at one thing at a time.
@zleap see, command and conquer, you have the expectation that you won't actually be the one shooting things. you're commanding troops, setting up defenses, etcetera. if i'm playing an FPS though i want to be the guy shooting stuff.
i know people rag on call of duty a lot for being "the same game over and over again" but honestly, the campaigns are usually a lot of fun. there aren't ever really moments where you're stuck in a spot holding down the area, you're always on the move and they have really great map layouts that guide your eyes naturally to where you're supposed to go and where you're supposed to shoot.