Have been using Buttontrader for several days now realtime. It performs very well. The only difference I notice is that the cancels are not as fast but this is to be expected. I primarily buy on the bid and sell on the ask and I must say that there is not a whole lot of difference between my fills simulation vs. realtime so whatever formula they are using is pretty good. I would encourage anyone who wants to papertrade to check it out.
www.buttontrader.com Buttontrader can be as complicated or simple as you want it to be. The first time I downloaded it I gave up on it as it seemed too complicated but after a couple of people here recommended it I tried it again. This time I did the tutorial and changed my mind.