I have a tomtom 300, which has been pretty good. I use it quite often with work and find it easy to use etc. and the post code search is very handy.
Only trouble with the portable sat nav systems is thieving mofos breaking in to your car. I always leave mine in the glove box as I cant be bothered to carry it around with me... I had my previous car broken into just because I had left the windscreen mount still attached... the tomtom was nicely locked away in the glove box, which they didnt manage to get.. but they stole the £30 screen mount... and caused several hundred £s worth of damage.. The coppers told me even if i had taken the screen mount of the windscreen, they would have still broken in, as they look for the marks left on the windscreen..