I got this through cydia. its called "IDA" but from what i read around the internet, it's essentially the same as sygic (just different name). Anyways it's only a trial for 7 days and i intend to purchase the european version when its available or buy tom tom when it's finally released. anyways moving on.......
I used this on saturday to travel to silverstone (race circuit) yesterday from brighton. A journey of about 120 or so miles. The software itself did a fantastic job, never got lost on my way there and back. The software is quick to re route if you make an incorrect turn etc.
The voice is loud and clear and can be heard quite clearly when travelling at 70mph+. I had the voice playing through the iphone speakers itself, so should imagine it would be alot better through the car stereo. One thing that does annoy me is you cant adjust the volume of the voice within the app itself. I have to go into my ipod, play a song and then adjust the volume from there!! Very strange if you ask me, but i havent found an option which allows you to adjust (the volume rocker on the side of the iphone only adjusts the ringer volume even when you have sygic open).
The software did lose track of my location about 2 times on the way to silverstone. This is mainly down to poor gps signal and not a fault of sygic itself. It did sort it self out within a few seconds so not really a problem. On my way back home from silverstone, I didnt have the same problem so it all depends on the GPS signal!
All in all, i think its a decent navigator, does what it says on the tin. I must admit, I'm still not 100% confident in the iphones GPS hardware. I took my tomtom with me just in the case the iphone would let me down. I think at this stage it would be wise to wait for tomtom to release their software and hardware. Would be nice to see how much that cradle can improve the iphones average GPS hardware.