I've owned all three of the big names.
Garmin GOOD point: almost never let me down. Found a good route; new the right turns. Worked, period.
Garmin GOOD: Alone of the big three, it doesn't care about spaces when you type street names and company names in. This is really important! I live on Oakleaf, which TomTom maps think is spelled Oak Leaf. It won't find "Oak Leaf St"; AFAItK that doesn't even exist. And if you're trying to drive to JoAnns, don't search for Jo Anns! So, Garmin wins for this point, too.
Garmin GOOD point: I found the Garmin's interface the most intuitive to use by far. Of course, that's subjective.
Garmin BAD point: updates were atrociously priced. Of course, if you buy one with lifetime upgrades for free, no problem there.
Magellan GOOD: has a super-neat feature: it beeps *just* as you reach turns, so if there are two closely spaced roads, you can be sure you're turning onto the right one without taking your attention off the road.
Magellan BAD: However, there were 12yo roads I traveled that it could identify (on the map), but not find (as a destination address). STUPID, STUPID GPS! Additionally, the computer voice pronunciation of road names is really awful.
Magellan BAD: got the address wrong about 50% of the time. It would show an odd-numbered house as being on the even-numbered side of the road (no big deal, but wrong), or show a destination as being 1/3 of a mile away from where it really was (a big error). My roommate had the same model (we bought at the same sale, so they should have been identical), but said he never had this problem, FWIW.
Magellan BAD: they sell advertising. So, you're driving along, following directions in heavy traffic, and suddenly a baar across the top of your already-busy screen announces "All you can eat lobster!", and the Red Lobster three blocks away appears on your map. That's a problem.
TomTom GOOD: Alone of the big three, it doesn't pop up ads. My Garmin (with limited map license) didn't do this, but according to dookie1949, above: "Regarding Lifetime Traffic; there is a significant difference between Tom Tom and Garmin. Garmin's traffic service includes advertisements for nearby businesses as you pass by while Tom Tom does not."
TomTom BAD: I currently have a TomTom. The software often gets confused, and locates my car in the middle of a block, instead of on the road.
TomTom BAD: The computer voice sometimes clips off syllables from road names it is pronouncing; this makes it harder to understand them. So far, not impressed, but it works better than the Magellan.
TomTom BAD: the zoom controls are backwards. If you want to expand the view (look closer), you press the leftmost or bottom button. That's reversed from almost every other mapping software out there, so it's confusing.