I bought an unlocked iPhone in the States and brought it down here and was able to use a Personal chip and it works fine.
The only caveat is that we couldn't find any of the micro chips, just the regular-sized ones. We bought a regular sized one and went to an independent shop who cut the chip to size with a template (don't remember which one - probably Galeria Jardin on Florida street has a shop in there that could do this).
You'll have to get the phone number balance charged at various kioscos - you can't have an account unless you have a DNI.
As far as the best company - I'd say it's between Personal and Movistar. I used to have Movistar but changed to Personal when I got my DNI so the family could all be on the same invoiced account. I don't see much difference - they all suck at one time or another.