i remove all the cats on my v6 and it sound better( Well, you need at least a cat-back)... But i feel a little difference with the acceleration and a better response to the throttle.
I did`t see any difference on my gaz millage (maybe your friend need new O2 Sensors!).
Bad side to be catless is the smell, you have no restriction at all so if you stand beside your exhaust you will notice it smell carbon monoxide. (i dont know how to spell it) lol.
Also you will notice a difference in your sound.. it will be a little more "growl" but a lot louder (Be carful with neighbord)
Hope this help!