Is there anything different between a iRobot Roomba and the Create? I want go start building my own turtlebot and playing with ROS but with the cost of all the parts I'm going to have to do it piece by piece. It's pretty easy to find cheap used Roombas.
Can you use a Roomba with ROS? Absolutely.
Can you use a Roomba with the existing turtlebot code? Most likely but it may depend on model of the Roomba since older models have a slightly different API. Of course even if there are API differences that haven't been accounted for it should not be too difficult to adjust the code to handle them.
Yes, there is a difference between the Create and an off-the-shelf Roomba. The Create doesn't have a vacuum motor or any of the cleaning brushes. And there is an empty payload bay where all of the cleaning stuff used to be. Additionally, the Create has an added microcontroller on it that you can push code onto.
But both the create and the Roomba let you control the robot directly over a serial interface. I think this API is the same between the Create and the 500 series Roomba. I am not sure about the 600 or 700 series Roombas, but i kind of doubt it changed.