How are longboards to ride on flat ground? Do you need a hill? will it act like a skate board on flat ground?
A longboard will roll a LOT more than a regular ol' skateboard, mostly due to the wheels (although there are other factors that play into it). If you really want, and can master pushing and/or pumping, you can REALLY get whipping along on flatground. You don't need a hill by any means, hills are just a fun side-effect of longboarding.
I was actually just out on Wednesday, on one of my longboards. I apparently got far too accustomed to snowboarding over the winter, and was subsequently trying to turn my board like a snowboard...It almost worked because of how I've got that board set up, but do note that I've never ridden a board that feels remotely similar to a snowboard.
I've got a longboard or two for sale, if anyone's interested.