My experience has been that protein shakes can be good meal replacements, but unless you've got the diet dialed in with real foods you're better off focusing your efforts there!
I think there are two approaches: Either use them to replace a meal, in which case you need to make sure it's nutritionally superior to the meal it replaced, or if you're using it to supplement your existing intake it won't get you anywhere if the rest of your food is crap.
If you feel you're eating well and just need additional protein, its probably not a bad way to go. Hope that helps, but its just my perspective.
