I have bought and like the ones from Strictly Briks. They have a nice heaviness and stiffness to them, the bricks grip well, and they're made so that you can attach bricks to the bottom or stack them. I've been thinking about getting some of the smaller ones to create elevated terrain. These are 10x10 in packs of 4 for $17,but they have different sizes and packs.
To me, I have no ethical problem with picking up cheap baseplates if they're made well. It's a flat piece of plastic! I'm just going to use the money I save buying more real Lego anyway :D
Edit to say they also come in a huge amount of colors, which is nice.