I have a Bug that I use with one bike and I have a milk crate on the other bike. Here are the pros/cons as I see them:

Bug:
Pros:
very neat
fits my stuff well
once attached to the rack its on until I take it off.

Cons:
When I want to take a change of clothes, books, and a few extras including my lock, its not so big anymore.
I have problems with the hook at the end of the elastic getting stuck at the junction of my rack/fender mount.
I have trouble finding a place for my lock...(I'm using the helmet holder currently)
After taking it off I have to un-velcro the straps before I'm off.
The hook sticks into my back when wearing it as a back-pack.

Milk Crate:
Pros:
Cheap ($10 including zip ties and hardware to mount my lights).
Lots of room to fit my traditional backpack full of clothes/books etc.
I can drop my lock in the bottom of the rack and not have to fit it in the bag nor fuss with it when I get to my destination.
When I'm there, I park & lift the backpack and I'm on my way.

Cons:
Mounting/unmounting: The high walls of the milk crate require a bit of additional flexibility to clear with my leg.
Also, when loaded it gets a bit more tricky since the weight of the crate is quite high, and any small tilt gets a lever effect.
Its pretty homely.

Hope that helps.