i only ever used em for that 1hp ^_^

if you get the vampire ability they heal alot more

You should still be able to if you go back to them and talk.

I think if you get a perk or something for joining them, this lets you get 20 hp back instead of just 1.Other than than that the blood packs are generally useless. 
 
Here's a link 
http://fallout.wikia.com/wiki/Hematophage

They are great if you finish the Arefu missions because they give you 20hp and no radiation so they're like purified water but you find alot more.

The vampire perk and option of selling blood packs to the vampire people make them have value but they really are not that special. Stimpaks have much, much more health and contain no weight. I would go with those instead.

Yeah if your a vampire it heals i think 20 or 50 depending on the type of vampire you are.

Go back to the tunnels and talk to the leader again. Ask him to teach you their ways and he'll agree to. After that you'll get a perk which allows you to regain 20hp from a single pack, much like purified water. You can also sell them to the vampires if you go back and talk.