Nothing wrong with the one piece as long as you won't be traveling with the bow. If travel is in your plans, buy the 3 piece. These are very nice bows, but they are not the wonder bows that many people suggest will throw arrows the same speed as other bows 10 pounds heavier. I am not sure what bows are being compared to the ACS to hold it out as being so much faster, but I don't get those kinds of speed differences with my ACS compared to my other high performance bows. Don't get me wrong, the ACS bows are smooth, fast, etc., but so are quite a few of their competition. The ACS is a darn fine bow that will do anything you need a trad bow to do, and it will do it as well as any bow you could buy, especially if you add the versatility of a 3 piece. Of course, like any bow, try before you buy to make sure it works well in your hands.
Allan