A hood's main purpose is to eliminate lens flare. When shooting in the general direction of the sun, you will get a streak of light in your image. A lens hood is made to prevent that.
A secondary use of a hood is to protect your lens from damage. It can keep you from bumping the front lens element on things. A collapsible hood won't help there. But if you are the type of person who is very careful that doesn't matter too much. And if you use polarizing or UV filters they will protect the glass as well.
Yes, the collapsible hoods should work just as well as the brand name hoods as long as they are made for the lens type. My 70-300G has a hood that is about 5" in diameter and about 7-8" long. My 18-70 has a hood that is only about 3" in diameter and maybe an inch in length.
If you shoot outside quite a bit, you would probably find a hood indespensible. I didn't realize the difference until recently when I used a hood.
at 12:04 AM on Mar. 8, 2009