I'm using a TomTom 550 on my GRiSO, also have a TomTom Rider on my Norge.
The user interface is easier on the Rider, but it's an outdated model and no longer sold or supported by TomTom.
The 550 is fast, it's a different sequence of events to plan a route out, but it's excellent quality. I also like that it's not monstrous, and can be easily taken off and put in your jacket pocket if need be.