Best thing about a head and neck restraint is that it holds your helmet up when you fall asleep, allowing you to nap comfortably- a worthwhile situation when you are racing after a group of late models who are trying to set the record for number of cars wrecked in one feature.
I can relate, so often being placed after the big-bore group (AS and ITE) that makes a habit of, if nothing else, thoroughly oiling-down the track before they're done...
Back to the orig question - I have both an ISAAC and a HANS (Sportsman w/ sliding tethers). First one is for racing; been using since, I dunno, I think '02 or '03... tested it once pretty big. Bought it for price but more importantly the higher expected level of side-impact protection vs a HANS - don't have too many occasions to spear a concrete wall in IT racing, but getting t-boned or other side impacts are much more likely.
The HANS is for work, so can't comment on racing use, though I wouldn't have any concerns about using it as such. I will say the pressure on my collarbones etc sucks, usually come away with light bruises after only one or two sessions! Yes, that's with the pads.
But the whole 38.1 single-release thing bugs me (WRT the ISAAC), and I'd have a hard time recommending that anyone else invest money in an ISAAC these days; not because the device doesn't work, but simply because it's a bit of a gamble, seems to me, that it will continue to be allowed even in SCCA... everywhere else has already legislated it out (by requiring a 38.1-compliant device). It just seems like the writing's on the wall, IMO, and the only question remaining is how many more years are we gonna get out of our ISAACs before we're forced to switch by the lawyers.
I expect that sometime in the future, within the next 5 years (solely my guess) I'll end up having to replace my ISAAC with a 38.1-compliant device. That said, I hate the idea of giving any of my money to Hubbard-Downing, rewarding their marketing strategy and tactics. OTOH, it's still the original and best 38.1 device, it'd seem. So this other new device (forget the name already, but it was posted here) is much more appealing... the basic function/effectiveness of the HANS, it would seem, without rewarding Hubbard-Downing... lower price is an added bonus, but I guess that largely reflects the profit margin on the HANS...
Of course, I expect that my next racecar won't have a roof either, so that will help with egress concerns...