I don't think there's any chance of a new Daemonhunters codex within a year, so if you want to field Grey Knights, go for it. Yes, it's an OLD codex. I think, barring Dark Eldar, they're now the oldest codex. So there are issues with it, but honestly, nothing that's too much of a problem.
As an army, they're still viable, particularly if you want to go heavy on the Grey Knights. Much of the really cool stuff in there (daemonhosts, assassins, etc) isn't terribly competitive under the new 5th ed environment, but Grey Knight Terminators in Land Raiders are nastier than ever. Inquisitors and Stormtroopers are overpriced but can be very useful.
So: the models are still awesome and the codex, while antiquated, is servicable. It does have some issues as it is a 3rd edition codex in a 5th edition world, but these can mostly be solved by downloading the FAQ/errata from GW's website (which seems to be down right now or I'd give you a link). This doesn't solve all of the problems, so I would heartily recommend the
Adepticon FAQThis is NOT official, but it is good and without any kind of decent errata from GW, it's at least a reasonable starting point in terms of how to deal with oddities (particularly how Grey Knights and Daemons interact these days with the new Codex.)