While for many people a standard car GPS is great for finding your way around the city (or country,) when you head off-road you need the more-detailed Topo Maps showing contours, features and tracks rather than just roads and lanes. The only problem is traditionally the off-road capable units were designed as hand helds for outdoor activities, so were of minimal use on-road to get there, meaning you really needed two devices.
Garmin recently provided us with a test unit of the Montana 650t which overcomes this by having all the roads with standard GPS auto-routing ability along with New Zealand and Australian Topo maps (the road maps also cover both countries). To get a comparison we also got the top-of-the-line Automotive (roads only) GPS the Nuvi 3590LMT.
On the road
The Nuvi 3590 is a pretty impressive unit - and at a retail price of $599 it should to be. It has a 5-inch 800x480 pixel  touch screen with spoken directions and street names, NZ and Australian maps, split screen lane display, and for some cities 3D view, and it also has a voice command feature. It comes with a windscreen mount with a powered speaker.
On test we were impressed by the clarity of the display and the ease of use of the touch screen. Slightly less impressive was the odd pronunciation of some street names, including pronouncing them as abbreviated to fit the display (So the Northern Motorway becomes the Northen Mtwy – said as “turn right onto Northern TWY”) but you could usually at least understand the instructions as they were also written on-screen.
We had a brief go with the voice commands, which certainly work, but have the problem of most such systems of not always being accurate in picking up the commands in a NZ accent, making them not fully practical for everyday use.
The Montana 650t has a 4-inch 400 x 272 pixel touchscreen display, which while not as amazing as the larger Nuvi, is still very clear and easy to read. It certainly performed well as an everyday around town GPS, making it very practical to do double duty (with the same pronunciation foibles as the Nuvi).
Off-Road
There is only one contestant here, the Montana. It does its job well, the display is great, and touchscreen technology makes it easy to use. Having the Australian topo maps as well is handy if you want to head across the ditch and head off-road.
The Montana is quite a bulky unit, but that is not an issue when vehicle mounting, and it can be used in either portrait or landscape orientation. Landscape works better on-road, with portrait being good off-road.
As with the Nuvi it comes with a powered speaker bracket, making the audio clear and easy to hear. It comes with a built in 5 MP camera allowing you to take a photo of an interesting point on the track which is tagged with its location. You can also remove the rechargeable lithium-ion battery pack and use standard AA batteries if you need to be away from a recharging source. Unusually, it comes with an AC charger.
Certainly for 4WD use the Montana is the better option, but it is an expensive beast (prices seemed to range between $860 and $1,000 depending on the retailer) compared to $599 for the Nuvi - but given that it doubles as a on-road GPS then you get pretty good value.