I purchased Copilot just today and I've had mixed results. In fact, the GPS sensitivity and map detail seem to be better on the iPhone's built-in Maps application (what I mean is Copilot's GPS accuracy and response seems slower than what I get on the Maps application). Perhaps Maps is using WiFi and cell tower triangulation to improve the GPS lock. Also, in my current location Copilot just displays a "near" street such-and-such message and no actual streets while Maps shows the full set of side streets for the complex where I'm located. It's not a new complex (a 15+ year-old mobile home park with paved and named streets). Maps shows all of these side streets, Copilot just shows that I'm off the main street in a completely blank area of the map.
I also had a few instances where the driving directions seem completely weird, like taking me in a circle around a block (multiple left turns followed by a right) instead of making a simple, single right turn to get on the proper route and street (and there were no one-way or right turn prohibitions). I suspect that what might be happening is that Copilot is making a circle-around when it can't determine it's true street location. This could even be a problem with the iPhone's GPS accuracy, perhaps Copilot isn't even at fault.
In any case, I certainly need more time with Copilot to determine just how well it can work, so don't take the previous "circle-around" comment as a confirmed issue.