I use the Nike+ and personally, I love it!
I was never able to get all those fancy watches and what nots to calibrate and work right and I eventually ended up giving up when I used them. My friend got the Nike+ and then I got it and we have both been using it since.
I don't use the pod in my shoe, just the GPS connection on my iphone. I know it's off when there's low GPS signal, but it also shows me that. Other than that, it seems pretty accurate to me. I compared it to Map My Run and they were pretty much the same.
They've also updated it recently so that the music continues when you take a break (I tend to stop mine when I'm at a light). It also seems the GPS works a lot better now. I'll be running near a canyon soon so I'll see if it's gotten better there as well. The time used to be inaccurate (I would get 6 min miles as well and I knew it was wrong...), but that also seems to have been fixed. I've been at the speeds I know I should be since the update.
Yes, it drains your battery quickly, but I always have a charger with me (in my room, backpack, and my car) so I don't really mind that.