i bought a used iPad 1 from a friend a few months back, at the time i wasn't sure i would find use for the iPad and was reluctant to spend $500 on something i wasn't sure about. so i bought the used iPad 1 and love it. but, over time the thickness of the iPad and the sluggishness of switching between more than one app at a time and web browsing really begun to make me resent it. my iPhone 4 even though it has a technically slower clock speed performs better because of the extra ram. I sold it and bought an iPad 2, the difference isn't negligible like a lot of people will tell you. you notice it with every swipe and touch. the speed, the fluidness, the switching of apps, page rendering, weight, thickness.
if i had to do it over again i would have just got the iPad 2. do yourself a favor and don't cheap out.