We put them in during our build, they worked out about EU10k more expensive than timber flooring, but I believe we were taken advantage of a bit there - i.e. we let the builder use his own preferred, local suppliers, in the interests of speed and a quiet life, but I think it would have worked out a bit cheaper if we'd gone with a very competitive quote from Quinn Prestress (in Cavan/Monaghan) who also provide a superior service in terms of engineering.
Our setup got quite complicated in terms of who'd sign off for the structural capabilities etc., our engineer and the hollowcore providers butted heads over a few things, there were delays, as per usual, things had to be altered, and so on.
After all that, I'm unconvinced of the benefits. Sound still travels between floors via the stairwell, and in some respects you may not want perfect vertical soundproofing anyway (e.g. hear babies crying, teenagers sneaking in/out at 4am, etc.
). Also there are the other problems mentioned above by earlier posters, such as drilling and so on.
I guess there's improved fireproofing by having concrete floors but I believe you can get high-rated ceiling plasterboards too.
To tell the truth, if I was doing it again, I wouldn't bother with the HC - UNLESS I could get it for half the price and half the hassle, which you may well do, I wish you luck with that.