I have two slicks and use both pretty regularly. One is a 4" Witherby-- great steel. Its sharpened dead flat on the back and with a radius on the bevel side. I use it for fine precise shaving and flattening mostly. Its a beatiful tool, so I tend to baby it a bit. The radiused bevel can be used "upside down"-- cutting with the bevel edge down-- to nice effect. Removes wood quickly, if need be, and also can be used to create a nice scalloped texture-- almost like a adzed surface. My other slick-- a 3" no-name "cast steel" is much better for gross wood removal, and also the one I'm a bit more willing to beat up on a bit. It is "rockered" on the backside as Tim is (I think) describing, which lets you pull your cut out when it tries to dig in too much (something that tends to happen when you remove a lot of wood quickly. It also gives you hand clearance for paring things with the grain in the middle of a timber. I have the bevel edge sharpened straight and square to the sides-- which also facilitates cleaning out occasional 3" wide mortises-- which I usually use for a tongue and fork joint at the peak, and in other special circumstances. I normally use a framing chisel to clean out normal 1 1/2" or 2" pockets. I've used the Barr slick some as well-- a friend owns one. Though it seems like a nice tool, I much prefer the older slicks like the ones I own. Its biggest limitation as I see it is that its way too light. So much of the slick's function derives from weight and momentum. You sharpen it up to a razor edge and focus on pushing its mass in a straight line. Inertia takes care of the rest. The Barr tool is so light that it makes you work much harder and the tool moves around a lot more. Its shape is also awkward in my opinion-- I think I would like it more if it were more straight rather than having the blade curve upward so much. A nice slick is a pleasure to use.
