I use the TomTomOne (1st Generation) and next to the latest TomTom130S recently available with street names. My gal took to using the TomToms like a duck to water and she really does not like technology. She prefers the text mode, where she can see the turn by turns as a list and listens only to the voice prompts. I use the graphics mode that everyone else uses that has the voice prompts and "street level" graphics.
I have very little experience with Garmin, brother in law has it and likes it a lot...but the TomTom seems to be easier to use for the novice and has more intuitive touch menu's, alternate routing and that ease of use sells it. While TomTom has a weather resistant version, big screen with full features and Bluetooth, the Zumo being made for HD motorcycles is more well known...and integrated name branding, mated accessories pricing match up when you get to the $600 mark to be personal choice and name prestige.