I've tried using 30 hellions before, they really are not good. I try to be optimistic and use units that aren't great, but hellions really just are in a rough spot. End of the day you're paying more pts than a tactical marine for a model that dies as easily as a guardsmen, and they don't really have an impressive punch to make up for that. It's a shame because I really do love the models.
All that being said, if you really like the models I'd say go for getting 10, I've been thinking of doing the same seeing how cheap they've become on ebay. You could throw 10 in a list and not feel too bad about it considering it's not a huge investment. Definitely don't try 40 though, they just can't make their pts back when you take that many.
The Night is Dark and Full of Terrors...