I have the tomtom one xl gb. After many journeys i would agree that its best on the bottom right corner of the windscreen, as its close to hand (not that i touch it whilst driving) and you barely need to move your eyes from the road to see it.
I run the cable behind the arch that covers the dials, then around a mobile phone holder, down to the socket. I don't really find it in the way. I aslo bought a four way cigerete socket adapter so that I can charge my phone at the same time