Laptops are perhaps poorly named, as very few people have mobile PCs sitting on their actual laps. There are several reasons for this: laptops can bounce around awkwardly without a flat surface to sit on, overheating systems can singe your legs (or at least become uncomfortably hot), and most importantly, there's no place to put your mouse.
The typical lap desk is a simple slab of plastic, which serves its purpose, but also works just about as well as a basic cafeteria tray would. Logitech's new $29 Lapdesk N315 goes beyond that by adding an all-over rubberized surface, as well as a pull-out side tray for your mouse (or to hold snacks, we suppose).
The off-white surface measures 14.5 inches by 10.2 inches--a good fit for a standard 14- or 15-inch laptop bag. It's designed for laptops up to 15.6 inches (which is the 16:9 version of the classic 15.4-inch mainstream laptop), but with a fairly small footprint; many 15-inch laptops will spill right over the edges of the lap desk.
On the right side, a smaller plastic wing pulls out to form an impromptu mouse pad. That measures 7.5 inches by 6 inches, and instead of the slip-proof rubber coating on the main surface, it's hard and somewhat slick, similar to a traditional plastic mouse pad. Of course, that mouse pad wing is only on the right side because that's how we situated the N315 on our lap. A simple 180-degree rotation and it could easily be on your left, with the only difference being an upside-down Logitech logo.