Need more info, like:
-for what use?
-for which bike?
Y'know, the basics :)
Otherwise there is no basis for opinions.
I have a single speed Jamis and a trek MTB
I just need somthing to get to and from work, my lunch and change of clothes for the gym are a bit much on my back...
personally I'm all about panniers. Anything really that makes a bike even more useful then it already is, will always be a plus in my book :D
They can be needlessly expensive however