This spell will put out over 500 points of damage per use, and best of all it is potion belt enabled!
The reason this is important is because the potion belt will always show the number of charges / uses the item has remaining. A fully-charged lava orb has 10 charges, so keeping track of how many uses you have left is essential.
The damage is light for lvl 61, but its damage is in addition to the other casting you're doing, your pet's damage, and possibly in addition to your merc or party. I'll take it.
Only disadvantages are;
a. only 10 charges, and they get used up pretty fast
b. Lore, so you can't summon multiples and always be ready to go during long battles,
c. Damage is rather light (500-ish),
d. No trade, so you can't equip your party with these.
All in all, I give this spell an A- for usefulness. Not my favorite spell, but I like it.