Dry the magma slimeballs and eating them gives you 30 sec of fire protection. Nice for getting those 1st few blaze rods.
Wow.
Are they properly farmable?
If so, this is going to make my Jet Fuel production chain a bit cooler...
[edit: Possibly the first reason i've seen to persuade me to leave 1.7.10 )
Something to (possibly) be aware of; fully grown slime trees actually spawn slimes on top of themselves (I know they do this for the overworld slime trees, not 100% about a magma slime tree planted in the overworld), so you may have a slime tree forest inhabited by Magma Slimes
given the apparent rate at which Magma Cubes spawn in Deserts if Etho's Let's Play series is anything to go by...)