Since this week seems to be all about lip products, let's have a look at this retractable lip brush by Sunday Riley. Retractable lip brushes are ideal for travel, as long as you manage not to lose them. The germaphobe in me can't avoid thinking that the part that holds the brush heads when you slide it back in is probably a breeding ground for bacteria, so I make a good use of alcohol wipes and sprays.
Other than that, Sunday Riley's lip brush is tapered and very pointy, making it great for precision work and for reaching the narrowest of corners, though Bobbi Brown's brush gives you a little more control. It's wide enough to coat the lips in a thin even layer of lipstick, and as such, Sunday Riley's brush is pretty equal to Laura Mercier's.
Bottom Line: One can never have too many good lip brushes.
Sunday Riley Lip Brush ($24) is available from Bergdorf Goodman, Barneys, and Dermstore/Blush.com. My brush was part of a gift bag from a press event.