Also, wet heavy snows will take them out. So figure you're going to have to re-build it every year, but its still cheaper than a quality steel and glass one.
One of my neighbors just uses hula hoops cut in half with plastic drop cloth over them to plant their tomatoes earlier. He's already getting fruit off his plants (and will probably remove the mini "greenhouses" any day now). There's also those water filled things that seem to work pretty well too. Tomatoes like sun and heat.
Also, anlther thing to consider is hydroponics. My dad built a hydroponic garden in the basement back when I was a kid and we had fresh beefsteaks for Christmas that year. With all the pot growing going on in the state you can find a good grow store in pretty much every town across Colorado and in general if its good for pot its good for tomatoes.