Here is a good comparison: http://www.mx5.net/products/wb/windcompare.html
I have a TC Windblocker, and it helps. However, if the wind is hitting you from the side (windy day), then they will not help unless you roll up the windows. I like it when it is a little too cold for top down, as I can install my blocker, roll up the windows, and be pretty comfy with the heater on. I only use it occasionally.