Leaving aside the unresolved argument over whether wood or plastic is more sanitary long-term, plastic cutting boards offer a few advantages over wood. They are generally cheaper than wood -- about $10 to $15, maybe a little higher if you buy a color-coded set at full retail price. They can be washed in a dishwasher (although the water must reach 140 degrees to be effective in killing bacteria). Plastic cutting boards are less decorative, so you'll likely be more willing to retire a scarred plastic board than a scarred maple cutting board. They are less prone to breaking (or breaking your foot) if you drop one. And if your kitchen is busy, color-coded plastic cutting boards can help you avoid cross-contamination -- you can have a red board for raw meat, yellow for poultry, blue for seafood, brown for cooked meat and green for produce, for instance (this is a trick used by some chefs).
Plastic is, however, lighter than wood and thus more prone to slippage while you cut unless the cutting board has a non-slip bottom (but that will usually mean only one side can be used as a cutting surface). Plastic cutting boards are a little tougher on knives than wood (but not by much). And plastic, of course, is less environmentally friendly than wood (although wood is less environmentally friendly than bamboo).
As with wood, there is some variance in the quality of plastic cutting boards. Cook's Illustrated says thicker plastic cutting boards are generally better. Watch for lips or ridges on plastic cutting boards. While lips are meant to contain liquids and ridges intended to hold food in one place, both can interfere with your cutting motion.
Experts, including Chow.com's Louisa Chu, say that polypropylene plastic cutting boards are of better quality than those made of polyethylene. Her review of plastic boards includes two of the three cutting boards also tested by Cook's Illustrated: the Architec Gripper (*Est. $20) and the Oxo Good Grips cutting board (*Est. $20), plus silicone cutting mats called Cut by Emblem Flexible Cutting Boards (*Est. $15 for four). Cutting mats are thin surfaces that can be placed atop a thicker cutting board to keep the main board sanitary. They are color- and icon-coded; the poultry mat, for example, is yellow and has an image of a chicken. You can place the red mat on the board, chop raw beef, throw that mat in the dishwasher, and then place a green mat on the board for chopping vegetables. Chu does note that they aren't as non-slip as advertised, and she places a damp paper towel between the mat and the board. "They're a lot cheaper and easier to store than four more cutting boards," Chu writes. "Plus they can be easily carried over to the stove and curved to funnel food directly into a pot."
While plastic boards are dishwasher-safe from a durability standpoint, most experts say scarred plastic cutting boards should be disinfected before being placed in the dishwasher because deep gouges can harbor bacteria and not all dishwashers get the water hot enough to kill them. If the board is deeply scarred, either get a new board or soak the old board overnight in a bleach solution.