Both Superb and Dreadful Equals Average...

I purchased my Tomtom 510 new from CircuitCity three months ago. I live in Boston, but travel frequently on business and take it with me.
Let me address the dreadful comment first. My biggest frustration is the accuracy of the maps for the USA. I have used the Tomtom in Boston, New York, San Francisco, Houston, Puerto Rico, Tampa and through Massachusetts and Connecticut. Since the primary use of the GPS is to get you from A to B, it is frustrating when the maps appear to be about five to six years out of date. For example, my home address is inaccurate by about five hundred yards. Since my house is a new development from about five years ago I can only conclude that it is plotting from very old map data. Yet my brother-in-law's car based navigation is exact, and uses a different company than Tomtom for map data.
To be fair, the system is about 90% accurate, but when it takes you to the wrong place (or declares 'you have reached your destination' and it ends up being another mile away) consistently, you lose confidence in this GPS.
The POI data is also mixed in terms of accuracy. On my travels I have found it to be about 50% correct. Things like ATM data is frequently wrong, 80% on garages, but it is usually right for restaurants. For those in the North-East, there is a Dunkin Donuts locator that is downloadable from the web site. Be warned that many of the stores don't exist! Tomtom will take you to the address, but one day I visited three houses before it got me to an actual DD. For this you would have to blame DD themselves I'm assuming.
I am not able to get my Bluetooth working from my Blackberry, but Tomtom did not claim to support my phone unit. I am hoping that one day that will work, because I would like to try their traffic and weather service.

So at this point you're probably wondering what could be 'superb'?
The unit is wonderfully easy to use, with a great screen, intuitive interface, sensible user options and really good battery life (I get approx 4.5 hours of use off one charge).
As well, the web-based connections for downloads, updates etc is simple and straightforward. Setting the unit up for trips etc. is great from my home desktop. I would assume that if you are reading this, you're internet-savvy, as is Tomtom as a company.
The range of voices is also fun and adds up to variety.
I give Tomtom special kudos on customer service. Like some other posters I began to experience problems with my mounting unit. Coincidentally the company proactively contacted me saying that the units were not meeting their quality standards, and they would be sending me a new one - which they did speedily and without fuss. I have written to them about the maps, and did receive a response telling me that there was an update due at the end of the year.

Given their handling of the mount issue, and their push into the US market, I am hoping that the map stuff is quickly resolved.

I do believe that the map issues I have experienced are fixable, which is why I am hanging on for the moment. I really like everything about this unit and my interactions with the company and it's web-based site. And in fairness, I can't speak with any authority on other manufacturers accuracy. Thank goodness for Epinions!