The E66 is Nokia's first E-series slider to be endowed with a built-in GPS receiver and can be assisted via the downloading of ephemeris data to help with speed. It took it about 70 seconds to get a fix from cold boot and it wouldn't work indoors unlike our Navman B10. You can read more about the Nokia E66's GPS performance from this dedicated article, but as far as GPS performance is concern the B10 Bluetooth receiver provided better performance overall. The advantage of having it integrated is you do not have to worry about carrying multiple devices during none-crucial trips to towns. It works with Nokia Maps 2.0 and Viewranger, as well as my friend's copy of Garmin XT (the application sucks though).
Applications tested that worked with the E66 are the built-in Nokia Maps 2, Viewranger (the latest version works), MGMaps (freeware - great app) and Garmin XT (the application sucks though).
I found the amount of the time to get fix from cold boot to be about on par with most devices with built-in GPS receiver, in that not so great but acceptable. Without resorting to A-GPS, the receiver took around two minutes to get a fix under a cloudy day (it has been cloudy the whole time so don't blame me for not testing it under the sun). Once a fix is gained it retains to the fix pretty well. I am not sure how many channels the receiver supports, but it is less than a dedicated external receiver like the B10 (which features 20-channel support).
Blue line: Navman B10; Red line: Nokia E66
To compare the accuracy between the B10 and Nokia E66, I used Viewranger to record the tracks I took home a couple of day ago while leaving both the B10 and E66 in my rucksack. According to the GPX files I analysed later I found that the track recorded via the B10 tended to be more accurate than the E66's recorded track. As you can see above the B10 (blue line) recorded the correct path I took when crossing the A411 where as the red was wildly inaccurate. Having said that the B10 isn't most accurate receiver, but hei it retails for only £20...
The screenshot above demonstrates the Nokia E66 (left) and Navman B10 (via E51)'s accuracy according to Nokia's built-in GPS Data application, based on both receivers situated around 2 feet from the window of our apartment. Despite being in door the B10's accuracy was around 11 metre, but the same application reported the E66 as having an accuracy to within 40 metre. Even then it reported a speed of 5 km/h despite it sitting pretty on a table. As far as accuracy is concern, the E66's unknown GPS receiver chipset (some are saying it is the same one that powers the N95) is behind the B10's SiRFstarIII chipset.
I do not drive so I won't comment about the E66's performance when it comes to providing turn by turn directions using programs like Nokia Maps or Tom Tom. As far as trekking is concern, if you do not need super accurate performance then I see no problem with using the E66 as your navigational platform, as long as you can dismiss its margin of error. As for using it as an everyday none-critical receiver, I see having one built-in as a no-brainer. The built-in receiver is a blessing and I find it handy walking around the back streets of London trying to find that one obscure store.