I have had the built in satnav in the Civic, Accord and CR-V. In the Merc I have just completed 10,000 miles with a Tomtom 750 which is mounted and hard wired using a Brodit mount. The Tomtom x70 series has a built in mobile phone (in effect) that uses GPRS data to get traffic and other info every couple of minutes.
Time, I think, for a comparison!
Basic requirements (simple nav)
Both are more or less the same. You just put the post code in and off you go. The TT is unresponsive for a while after boot as it does other stuff (getting traffic data) but on the move both will get you there fine.
The TT arrival time is based on statistical speeds for the roads in use. This is clever, so long as you drive at those speeds (you can't tweak them). So for me it's inaccurate because I drive at 70 on the motorway, and the statistical speed for 3am on the M1 appears to be about 80. The Honda has adjustable speeds for different road types which I got to be more accurate.
The TT is hopeless at roundabouts and junctions. Because it is GPS-only there is a lag in its calculations and the "next exit" can come a long time after you have actually exited the roundabout. The Honda uses yaw and actual car speed, so is deadly accurate on roundabouts.
The Honda maps are less accurate than the TT (which has constantly updated maps via user feedback).
The database of POIs on the Honda is vast compared to the TT, but the TT allows you to download third party ones if you can be bothered. The TT includes speed camera POIs, which are hopelessly out of date (and I subscribe to the premium service). It also has a Google search function, so you can search for the POI and if Google finds it you can then navigate there. I managed to get this to work once, but it has failed several times. The TT also can show you where the nearest cheap petrol is. One of the best functions on the TT is that it knows and displays the speed limit on nearly every road. And you can set it to beep if you exceed it by a certain margin.
If you are on the motorway and you are travelling with children, you will want to know where and when you are going to pass the upcoming service areas. The Honda does this with one button push - I have yet to work out how to do this at all on the TT. The TT doesn't even display motorway junction numbers.
For those of us who like to use a satnav like a map, it is nice to be able to display in 2D. The Honda is brilliant at this - you can quickly set the scale or move around laterally using hard buttons. Screen re-draws are quick. The TT is almost unusable. When navigating you can only zoom in and out (no lateral movement) and the soft zoom buttons either don't work at all, or work very slowly. You soon learn that to zoom out, you have to do one zoom in first. If you miss the tiny zoom button, you can end up in the "navigate home" screen, which has no back button. The 2D display in this mode is hopeless too, with no real detail and no differentiation of on-route traffic compared to off-route traffic (which the Honda does).
To move laterally in the TT, you have to go into map mode (several button pushes). This mode usefully shows a 2D map of the position you last left map mode. So first job is Find, Next page, Current position to show where you are. Then it will zoom in and out, but because the zoom buttons are very small and right next to the zoom scale, you often get the whole of Europe, or the surrounding 10 yards. What's worse, is the screen re-draws take forever. Then, for an early heat attack, try scrolling around. Re-draws take ages, and often the finger-swipe hardly registers so you didn't scroll like you wanted to. Then it leaves a strange blue icon in the corner of the screen and for some reason when you zoom in it doesn't zoom to the centre of the screen it zooms to the blue icon. My advice is that if you try 2D work on the TT, keep the window closed to prevent an ill-advised angry lob.
The Honda uses free TMC. The accuracy of the TMC service has been variable, but is generally quite good. The re-routes are quick. The display of traffic on the 2D screen is excellent (for you to work out your own re-route).
The TT gets data
from the built in mobile, and this costs £80 pa. TT collect data from lots of sources (including, I suspect, from other TT users stuck in traffic). So the data is better, and includes how long the delay is. The re-routes can work out whether it's best to stick it out in the queue, or take the smaller roads. If you buy your satnav primarily for traffic avoidance then the TT is the one to have. But both still suffer from "stuck" events, or events that just never appear. You can see the TT live data here
The Honda maps are too expensive, so for financial reasons become even more out of date than they should be.
The TT does not use its light sensor, so only dims at computed dusk. Minimum brightness is too bright. The voice control doesn't work. When you arrive at your destination it doesn't cancel the route. The sound is weak and doesn't mute the stereo (like the built-in Honda). You have to remove it when you park the car.
The Honda system is intuitive, responsive, reasonably accurate and a pleasure to use.
The Tomtom is frustrating, slow and unresponsive, but has slightly better maps and Traffic.
I swap between the two almost daily, and much prefer the Honda system.