I bought this add on for the PSP so that I could watch films during my frequent trips to Spain and have a ready-to-go Sat Nav when I got there.
The Sat Nav function performs perfectly fine despite a slight delay initially whilst the Go Explore software works out your curent location and which satellites to use. Some reviews have criticised the amount of time this process takes however, I have found very few other Sat Nav devices which operate any faster. Locating destinations is very straightforward and the route planner works everything out for you seemlessly.
Map detail is pretty much spot on - very easy to follow. One way streets and speed limits are clearly indicated, and all driving actions are called out well in advance of any manoeuvres.
The only gripe I have with this product is the price. A top of the line, dedicated Sat Nav may cost you around £150. I imagine the bulk of that price might go to the actual hardware. In the case of Go Explore you already have the hardware, so why the price tag of £100???
In conclusion, this is a perfectly fine Sat Nav with some interesting features. Whether you think your money is better spent on a dedicated machine - I'll leave that up to you. Personally, having one device that plays music, and films on the flight, and gets me to my hotel in my hire car when I get there makes this the right device for me.