I occasionally use protein bars and shakes as meals, but rarely as more than one meal. Whenever I've tried to do it for more than one meal, I start feeling like I'm in diet jail.
For me, I think it's a "variety" thing. I get bored really easily, and for me, food boredom triggers cravings.
I do use shakes as meals more often since I got a Rocket blender (like the Magic Bullet that is advertised on informercials). I use a vanilla whey protein powder (when I run out, I'm going to buy an unflavored whey protein) and then add skim milk or low-fat yogurt and fruit.
By making my own shakes, I can increase the variety.
More than a few meals a week though, and I think I'd go batty.
I tried doing protein shakes. Really, they just weren't as filling as areal meal or me and I knew it was aecting my nutrition Once in a while was really handy, though
I use EAS Myoplex Lite ready-to-drink shakes for early breakfast (5-6am) and for those times when life gets in the way of having a healthy meal. Even though they aren't as filling as a regular meal, they are nutritionally balanced, and help me stay on plan.