I've had the 550, 660, 590 (with which I have dabbled with 595 software) and I have the XT.
For sheer usability, the 660 was (in my opinion) the best of them. By that, I mean that the way that it navigated and how it behaved when going off route, was so much more predictable. As long as you returned to the magenta route, it didn't care that you didn't ride through every single route point. It just got on with it and directed you from there. The battery lasted for hours, and it had a speaker in the unit itself, which made setting it up in the comfort of you armchair very easy. But it had its foibles. Figure-of-8-routes could be interesting if you were not paying attention, especially if the cross over point shared the same section of road. It would happily navigate you down either direction. Turn back and take the other direction and it would happily navigate you along that. It caught a lot of people out.
My 660 used to require a reboot every now and then on the route - the map would not redraw - I knew when it was about to do this - it would start to go slow, as if it had run out of memory. Resetting it would solve the problem, until the next time. One thing you will notice with any of the later units, is the speed of the processing.
The 590, 595 and XT will not work reliably with Mapsource. You would need to make the move to Basecamp. But that is not the big issue that it was when I had my 660, when Basecamp was really bad (my opinion). And there are lots of on line videos to help nowadays.
The 590, 595 and XT all have the Trip Planner App, and this does things very differently from the way that the 660 operated. When I moved to the 590 from the 660, it was a choice. I traded the 660 in with Garmin, and for a few weeks I regretted it. Seriously. I really thought that I had just thrown away a few hundred pounds. The 590 didn't behave as I expected. I hated it. But once I worked out what it was doing, everything became predictable again - but it was a very steep learning curve.
The 590/595 has a few serious gotchas which will make you believe that buying it was a bad move - but once aware, you can make them work for you. I like the wiring that comes with the 590/5 - but I liked wired communication. It has power, mike in, sound out, USB charger, all long enough to reach towards the rear of the bike. But serious flaws are the changeable battery. It has never had much of a life when away from a power supply. 20 minutes - often much less, after which it dims the screen to the point where it is often unreadable. I've recently changed the battery with a new one from Garmin, That was better, but it still doesn't give much time to do things with. It doesn't have a speaker, so testing out phone connections and music and volumes is impossible unless wired in. And the screen is not very bright. You have to remember to check the brightness setting, but it is a weak point.
Although the 595 has some better features, it is essentially the same unit - and indeed the software for the 595 will work in the 590. Some versions of the 595 do not allow it to be used in portrait orientation. I think this may be a regional variation, reading the manuals. I have never felt the need for tyre pressure monitors - not at those prices anyway, so that would not be an issue for me.
The XT has its software very much developed from the 595, but improves on much of it, and takes away some of the annoying features. Currently the XT has a few things which make it behave oddly, but they keep updating the software to address those.
It has no wired connections, other than a power lead. Everything is done by Bluetooth - which means if you have rider/pillion communication, you may well be restricted to what connections are made between the Zumo and the rider / pillion devices. I have a Bluetooth module which plugs into my wired autocom, and that makes all of the connections for us. The XT continues to offer the smartphone links for phone calls, music, traffic and weather data that was present in the 59x series, and that set up works well. It seems to be capable of getting traffic date through the power lead, as well as from the smartphone, but I haven't experimented with that yet.
But the big plus points with the XT - the battery life away from power is almost as good as the 660 was. The screen is large and it really is very bright and clear, and it introduces some additional mapping displays - which include satellite (birds eye view) imagery. It removes the start of route hassle of the 590 and 595. Odd screen display could result in selecting the wrong start point of a route which would end up with you following a different route from the one that you plotted. That feature still exists, but more prominent is a new option to start from the closest point of the plotted magenta route from wherever you are. It now seems to do an excellent job of finding this - although you need to ensure you update the software as soon as you get the unit as the first two releases did not get this right at all. It talks to Garmin by Wifi - for updating software and maps - but not for transferring routes, unfortunately - not that I have found anyway. That is still done by wired USB connection.
The 590/595 were very robust in construction having a rubberised surround that protected the screen. Drop a 59x and it bounces. The XT has the appearance of a smartphone design. The screen is very close to the edge. Maybe Garmin have designed the case using shock absorbent materials - but my personal feeling is that I really wouldn't want to drop this at the side of the road.
I think the XT is well worth getting - but moving from the 660 to any of the 590, 595 or XT, you will have the same steep learning curve to climb. But really, once you have the idea of how the basic routing points behave, you have cracked the main issues. Personally, with the XT now available, I wouldn't give the 590s a second glance - it is cheaper for a start, and (apart from the pressure sensors) offers much more. I still have my 590 / 595 depending which software I have on it - and I am putting the XT through its paces, so it is not actually fitted to the bike yet. I have discovered a few oddities, reported a few issues which have since been addressed, and noticed a few other things - which it turned out also existed on the 590 and 595, but I never noticed. Must be the brighter screen !
Read this - the pdf attached to the first post. I wrote it for the 590 and 595, but the first two sections are broadly relevant to the XT. It also contains some links to Basecamp videos which illustrate some of the routing features.
A detailed analysis of how the later Zumos (with the Trip Planner App) work together. This is the result of many hours of working out how my Zumo 590 and 595 behave with routes created in Basecamp and results in some recommendations for trouble free route planning. The on-line Issuu version...
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