I use their foundation and cheekstains exclusively for foundation and blush because I'm allergic to everything else I've tried (I have crazy sensitive skin and I can't even narrow down what specific ingredients I'm allergic to). I love their Smooth Operator foundation - it's light, which I love, and Facade perfectly matches my very pale skin. It's also got SPF, which is great! I like their gel cheekstains a lot - I don't have issues with them being sticky, and they last all day on me. My faves are Dollface, Cloud 9, and True Love. I also use their Clean Slate primer, and it works great for me. I'm very loyal to them for those three products, because nothing else I've tried has worked for me!
I also have eyeshadows from them, and I really like them. Since they reformulated them a few years back, their eyeshadows are smooth and blend easily, and I find that, over Urban Decay Primer Potion at least, they show up very much like they do in the pan.
I'm a lipgloss junkie, so I have their lipglosses, too. The normal double-ended glosses are nice - smooth, hardly sticky (much less than MAC's lipglasses), and nice color, but I adore the Inside Out gloss. The stick-factor is similar to Lipglass, but they're soooooooo moisturizing. Most of the Inside Out colors are pretty sheer, but R&R isn't.
I dig their brushes, too. Their flat-top blending brush (discontinued, unfortunately) is so soft and easy to use. I got a few and have given them as gifts, and every person I've given one to has come back to me a couple months later and said they loved the brush and use it all the time.
Their double-ended eyeshadow brush is pretty nice, too.
Because of my allergies, I have a bunch of their stuff - I'm allergic to only a very few items from them, which is something I haven't been able to say about any other line.
Added: I've tried their Yellow Mellow, and while I find it easy to use and it doesn't irritate my skin at all, I (very fortunately) don't have much in the way of redness unless my other allergies (cats, dogs, the outdoors, etc) are acting up, but it works nicely on my mom. I find it works best for me (and her) with cream first, then foundation, then powder.