I tried the prepaid when I first moved here and it wasn't too too bad for street navigation and email. Even with good credit (USA), cards and cash on hand, none of the companies would let me sign up. Even when I offered to pay upfront for the year.
Thankfully, a good Porteño friend of mine, took pity on me and put me on his plan. Then I just paid him the year in advance.
Obviously, that's not a solution for everyone but it worked for me.
Oh yeah, I'm on MoviStar, and the speed has been about 450kbps up and down. That said, the wireless cells are directly across the street from me on the other roof. I'm probably being cooked from the inside out as we speak....but I have great 3G.