I have the TomTom GO 6100 which is a little larger than the 5100 but the same features I think. Both these units get very good reviews. I'm very happy with mine. Have previously had Garmins.
I realise there are loads of options with smartphones and tablets, with others have described in this thread, but a dedicated satnav is still what we prefer.
In response to the OP, I have testing TomTom Go App (v1.10, Android), Waze and Google Maps. With the TomTom app you download the maps worldwide unlimited and have traffic unlimited* . I have UK & Ireland and the whole of Western Europe downloaded. Works great and the camera and traffic information is superb. I believe it has saved me lots of time and fuel.
Waze and Google NAV only work with a data connection, you can download the route with Google NAV but you have to remember to do that, I tend to forget. You can use it for typical routes you have already done. So home and work are OK but search for a location that is new you'll find it asks for a data connection after searching your off-line data. No camera location with Google but traffic is pretty good. Waze has cameras and traffic and it's as good as Google Nav but couldn't say better or worse.
TOMTOM app can always plan a route but you loose both traffic and camera locations without a data connection.
Question for VD: does the Tomtom 6100 loose the camera and traffic or is this part of the SIM deal that comes with it?
I was considering the new 6200 provided it gives the camera and traffic constantly regardless of a 2G/3G/4G signal?
Before anyone tells me I'm incorrect, try disabling the 3G/4G connection and plan a route see how far you get.
Depending on the reply from VD I'd buy a dedicated TOMTOM and use that for location where I know data is lost and for local UK use one of the apps from the mobile.
As for data usage from 22 Sept - 23 Oct TOMTOM used 10Mb, Waze 75Mb, Google Maps 105Mb.
I've used Google maps more than Waze but often check routes and traffic with Tomtom.
JabberWocky if you want more info you have my email and or PM me
*Only as long as the device is still supported by TomTom.
EDIT:
I remember reading somewhere that none are very good with road closures since at least the TOMTOM relies on operator input where as traffic is device based.