Nikwax liquids are excellent, not least because you can apply them to wet boots which makes them very handy in the field. Until recently I stuck doggedly with all leather boots, and will probably go back to them when my current Brashers wear out, and used mink oil. Chris Brasher, back in the days before the Brasher company started selling their own waxes and creams, always recommended any good wax polish and I'd suggest the same for leather. Mink oil does tend to soften and stretch the boot. Fabric/leather certainly benefits from the Nikwax treatment.
Nikwax supposedly doesn't affect the Gore-Tex either, but after long experience with different boots I remain entirely unconvinced by Gore-Tex linings. The material simply breaks down too quickly, even in quality boots like Brashers where the membrane is protected by leather inside. Having said that, my current boots have just such a Gore-Tex lining, but very often I wear non-waterproof shoes anyway. Good wool socks provide adequate comfort even when soaking wet.