Let's go on an ACID trip, baby! As a player and a DM, I think acid-based spells and attacks are great, simply because most players tend to have fire and lightning protection. Spells like acid splash, acid arrow, and acid fog are no save/no spell resistance, so it's not big damage but it is almost sure, constant damage.
Acid can also be used to melt locks and otherwise facilitate escape from some situations where other energy types are not effective.
Help me, I'm melting!