Free trials of MMOs are terrible. I did the RIFT trial, and I upgraded before the end just so I could actually talk to the people I knew who were playing the game.
Also, level 20 really doesn't take long. I could make it to 20 in about 8 hours. Racing to 20, mind you, but even if you stop and take your time, level 20 won't give you much of a taste of the actual game. Especially since the classes are boring as hell before that point.