Most beads are plastic, but every once in awhile you'll run across some heavy glass ones - these are definitely keepers. Most of the Krewes now have their own brand of beads with plastic medallions attached bearing their logo.
The size of the pile of beads around a person's neck is generally a good indication of how much success they're having, or more appropriately - how much attention they manage to draw.
Krewe members generally spend hundreds, sometimes thousands of dollars on beads alone. A rider will generally go through at least 100-200 gross of beads along a typical 4-6 mile parade route.