If you get a Tomtom make sure it's not one of the newest Tomtom Go units (400, 500, 600, 5000, 6000). The software basically isn't finished yet and the whole series are a massive beta test.
For your phone, make sure the compass is calibrated and get an app that resets the GPS cache. If it's on Android GPS Status and Toolkit is woth a go. Something like ActiveGPS or GPS Satellite Widget is also handy for ensuring your phone keeps a lock.
Seriously, try getting your phone's GPS working reliably because paid phone apps are up there with the best standalone satnavs now for a fraction of the price. Windscreen or dashboard mounts for phones don't cost much either.
If you just need to find your way, Google Maps navigation or Navfree are perfectly adequate. No need to spend more.
I use satnav for traffic avoidance rather than route finding most of the time. For traffic avoidance - it pretty much has to be Tomtom HD Traffic for road coverage. It's great at detecting congestion (and has received a major upgrade in the UK during the past few months). Unfortunately Tomtom doesn't always have info on road closures from local authorities so you may still bump into the odd road closure (if you're route is covered by the Highways Agency, Transport for London, Traffic Scotland or Traffic Wales, your'e fine though).
The cheapest way to get Tomtom's HD Traffic is the Android or iPhone app, Navigate 6. It's about 1/3 of the price of the Tomtom app and routing around traffic is better than Tomtom itself too.