A friend did a lot of research and got the Caribou 36L side case system. I'm very happy I went with them too. The 36L have particularly nice latches. I've heard other cases' latches are difficult to operate and hurt the fingers. These open and close easily, and come on and off the bike like suitcases with the greatest of ease.
I've dropped the bike on each side with very minor damage to the cases. I'm pretty sure metal cases would have been dented and scratched until ugly. These Pelican cases can be wiped with Back To Black until they shine and scratches are rendered invisible from afar.
Since they're slightly flexible, one can over-stuff them once in a while. Yet they don't deform permanently, and remain water tight. Mine have never shown any signs of leaking. They also have pressure equalizers.
I highly recommend going with the snap-in liners that they'll install for you. They significantly multiply the usefulness of the cases. Besides keeping things from falling out, they protect the hinges from opening too far. I can take home up to 8 bags of groceries by leaving the cases open (supermarket is a short distance away).
Disadvantages: helmet doesn't fit in (so I keep a helmet cable lock in there); liners often pinch and have to be pushed in when closing the cases (not too big a deal); the latch hook snags on things being removed from the case (annoying); the locks are primitive (I found my key to access the sprinkler boxes in a local park ;D).