I got the Nirvana-2 goggles a couple of weeks back. I haven't had as much chances to play around with them as I'd like, but here are a couple of observations.
The difference to diving without any facial equipment is of course huge. You can see almost as good as on land or with a mask. But you have to adjust the lenses properly first and this can be a bit of a task to do. But I would encourage people out there who have tried fluid goggles and had a bad experience to spend a few hours playing around with the adjustments, because in the end, the image is quite good.
It is not quite as good as a mask, though, so be aware that you are making a compromise. Getting a better dive reflex, freeing your hands from equalization and removing the need to equalize the mask versus smaller field of vision, slightly magnified image, slight discomfort and added complexity.
It can be a little disorienting at first, and you have to dive more than a couple of sessions to get used to them. Also, by added complexity I mean that with a mask, you throw it on and you're out. But with fluidgoggles, you have your bottles of saline etc to worry about, which can be annoying if you're not diving from a boat or platform.
As for at what depth is it necessary to ditch the mask? Well, world records are being set with masks, so...Personally I haven't found such limit yet.
Bottom line is, I think especially for recreational divers the Nirvana-2 is a huge improvement over the old one because of the better heat insulation and adjustability. But I would not necessarily automatically recommend them to every body. For competetive or "serious recreational" divers they may be a more obvious choice, and for FRC diving I would say they are excellent.
Let's just say if you have never felt that the mask is not doing everything for you, then, stick with the mask. If you are a dive geek (like me) and simply must try every gadget, then get them. If you are a competitive diver who wishes to get his hands free, then most definately get them...
I haven't been out with them much, but from the little experience that I have, the cold insulation seems to be almost as good as a mask (of course, not totally, as it has no airspace). I could stay down at some very cold water for a minute or so without mentionable discomfort...
