Hello Carl Lindh (Kvick),
Well by experience I use de XDJ-RX now for more then a year with USB Key analysed in Rekordbox 3.3.0 and it works fine no comments to say to this peace of gear which is wonderful.
You can make very good partys with it and getting the most out of it with FX, EQ, Beateffects it's superbe. But then you get limited in making your own beats and neede a four channel.
I've purchased the Traktor S8 from NI....very good gear but had dropouts,glitches with it and couldn't fix the issues even with a Very powerful PC.
What I've done is...I tried the xdj-rx in midi mode with rekordbox dj....and I've done a few gigs of 6 hours a few times ....without having any issues with Rekorbox DJ.
So I'm thinking to purchase the DDJ-RZ with Rekordbox DJ .....and hopping there will not be issues with it.
Good Look