One word: buy.
They're versatile and easy to use with a brush or air brush. I used the copper on y'varha's plasma flamer barrels and have a gorgeous heat burn effect now. I used the cobalt over black on the jet engine outlets and it's done a lovely blued steel effect, and the same over silver base coat gives a very subtle weathering effect on the inlets.
They can be mixed too, with a silver base coat, a layer or two of copper and a layer or two of cobalt has done a beautiful effect of dirty, heat damaged metal on the weapon vents.
I also gave my stealth suits a quick coat over their existing paint job, and now they shimmer and ripple as a really good example of a stealth field.
The main thing to watch out for is what finish you apply the paint over. Gloss undercoat with give a smooth shimmer, matte will give you a glittery effect- which looks good in its own right but was unexpected.
A further, non tau tip I found was that if anyone plays historical or fantasy games, a quick ***thin*** coat of the cobalt over chainmail with a leadbelcher and gloss nuln oil wash undercoat gives an effect exactly like you'd see on blackened maille armour irl (I'm a medieval reenactor so I've been trying to get this perfect match to what my real armour looks like for ages!). Likewise, the other colours applied over silk clothing on models gives it the correct lustre
As to the packaging, they come in dropper bottles which is massively convenient for a gravity feed airbrush but is darn inconvenient for brush painters.
All in all, they're expensive but worth it, and the fact that they are currently only available in a set compounds the problem. I've spoken to GSW and they'll soon be available individually. As soon as this happens there's literally no excuse not to get them, and really bring your models to life!
Limitations? You mean you don't want Fido the kroot hound as a character?