I tested around a bit tonight and here's what I found out. Some of it is an addition to what ZarDoom wrote, some of it contradicts his post. I was trying on the Training Dummy without any buffs.
-The worms have pretty much 20% of my health
-Their damage scales with gear
-When they pop they heal everyone around for about 1% of the DKs HP for each stack of Blood Gorged
-They even heal players that are not in your group or raid
-They usually pop around 5 to 10 stacks, I've never seen them with more than 10 stacks. They seem to pop on their next hit. Lowest was 3 stacks
-They can pop if you're on full health
Fun fact: the worm heals itself from its explosion