I think it's a great combination. For me, however, when I smoke weed (I do opiates AND smoke every single day) I get very tired *if I smoke more than a few bowls (of an indica heavy strain)*. That being said, if you are easily sedated when you smoke weed -- and dislike that sedation-- then smoke weed with opiates only in the evening OR if you do not need to drive anywhere (or do anything where one should be alert etc).
I just did some oxy and H, about to load up a bowl right now! Thanks for reminding me

haha.
EDIT:
Why is it a great combo? When I smoke weed (either before, during, or after opiate use) I need LESS of all other substances: opiates, benzos, etc. It REALLY intensifies the calming feelings, the relaxation, focus abilities, and the nod. It really allows me to stretch out my (opiate) stash a LOT longer. I have saved significant amounts of money previously wasted on H or other opioids since increasing my marijuana intake. Negatives? TOO much sedation if already a bit lethargic... and that's about it.