I tried wearing my pump on a lanyard for a while. I stopped, though, because it kept swinging in an irritating fashion, and my neck didn't appreciate the extra load.
I have a 640G, and it's great. The only downside is that it doesn't have an 'upside-down' feature... I wear it on my belt, so it's always handy when I need to bolus or set a temp basal. But the display is 'right-side-up', meaning that it's great for anyone who's three feet high and wants to look at my pump, but when I curl over to do anything, it's upside down. So I actually wear it upside down, so that I don't have to unclip it from my belt.