I much prefer a built-in system for two reasons - first, it's integrated with the stereo system so you get very clear speech and it fades the music to the back speakers while talking; and second you don't have to remove it from the car every time you get out to prevent it being stolen.
Saying that the key disadvantage is that it's fitted to one car so if you have more than one it's maybe not the best solution. For that reason I bought Tom Tom Navigator 6 for my phone, and it works surprisingly well (running on a Nokia E65 which is one of the latest Symbian S60 v3 phones). I installed it onto a 2Gb micro SD card, and got maps for all of Europe as well as North America -- it was an absolute godsend when I was in the US, such a breeze compared to when I was there a few years ago and had to navigate by map in the dark. It uses a bluetooth GPS module so just switch it on and throw it onto the dash, it picks up the signal very quickly and has never dropped out in my experience. Because I already have a bluetooth car kit, it automatically connects to this when the ignition is turned on so any navigation announcements mute the radio and come out of the main speakers.
Whenever I'm in the Omega I still use the built-in navi, even though the maps are now out of date compared to the Tom Tom. This has only been evident once though, when it told me to take the 2nd exit on the roundabout which had actually been converted to a flyover bypassing the roundabout completely! However the Tom Tom comes highly recommended, especially if you already have one of the latest Symbian phones with enough processing power to run it at full speed.