I avoided HO/VHO namely due to heat. I'm really not a fan (no pun intended) of having to deal with excess heat from bulbs and ballasts. That said, I may play with some small T5 HOs for my mini reef.
Well, that and because we had the salesmen for the NO T5s come in the door and I get them close to wholesale.
He's now said they're going to be selling HO T5s (probably actinics and 6400K again) so I'll give that a shot in my reef.
As for ODNO - tried it. It's not worth it (on T5s anyway). I ran 2 bulbs side by side. One was NO and the other VHO (three leads on it).. The light increase was marginal (not even double), and the heat increase was .. well obvious. :P
Thanks for the link.
Oh, and an update on the heat measurement.. I wasn't able to get a proper reading of the ballast temp, but approximating I would probably say it runs around 50C. Well within the acceptable range but perhaps warmer than you'd want stuffed inside an unventilated hobby box.