The PowerTap never leaves me wondering if its right. I've seen PTSLs for $450 used, not built. So, for about $550 you could have everything.

Regarding the losses on the 5s power, I believe that an accurate power profile is important to assess your race limiters. If it's short-changing you "a little" on the sprints, the you really don't know where you are.

Different riding positions will also impact the readings. I spend a little over half my time in the drops. So what position would I calibrate it too, drops or hoods? One bottle or too? My tight jersey or my looser one?

That said, the ibike is an amazing device, and I do believe them that it's gotten a lot better. One thought is that getting an ibike with the later intention of getting an ANT+Sport power meter might make sense. If you combine the ibike with a Quarq, SRM, or 2009 PowerTap wireless, you've got a portable wind tunnel, that's really accurate. That's just cool.