UD used to be my favourite makeup brand, but they turned against me, they insisted on d/c -ing everything I really, really loved *sob, sob* - (RIP lip inks, cream eyeshadow, metal face cases, etc...)
Their face cases are great value if you want to try a range of things out (though I'm still weeping that they d/c the older, slimmer metal cased ones, sniffle)
Their lipgunks are great, highly pigmented glosses (more liquid lipsticks, really) with candy flavours that I think are great (but guess what, I think they're getting discontinued).
Their xxx cooling lipglosses are.... OK. They taste nice, but are all very very transparent. If you like juicy tubes (I don't), you might like these.
Their single "standard" eyeshadows range from superlative to "too annoying to use" IMHO. I love the shimmer ones like Shotgun, Shattered and Vert, I hate the "glittery" ones (Uzi, Oil Slick, Kiddie Pool, Exhaust etc) as the glitter does not stay on your lids at all. They depot brilliantly into MAC palettes.
Their 24/7 eye pencils are a nice formula and pretty colours, but don't stay any better than any other eye pencil I've tried.
UDPP is great for fixing eyemakeup, but it seems to irritate my lids a bit and makes blending a good deal tougher. I've used my bottle up (I used to keep it for special occasions) and am undecided as to whether to repurchase. I've never heard of anyone else getting itchy eyelids with it, though.
The best thing in their range at the moment IMHO is the glitter eyeliners. I have 4, and would particulary recommend "Baked" and "Mullet". They're wonderful.
Their mineral makeup is very nice indeed and cheaper than a lot of other high end mineral foundations, but I prefer liquids tbh. I got a sample at one point and was pretty impressed with it.
I reckon MAC beats UD hands down for value for money (at least in the UK) and I only buy UD if it's on sale (most things seem about 15% more expensive than the MAC equivalent).
I don't have much NARS, just one eyeshadow - "Night Porter", a dense black with emerald shimmer - and it's beautiful.