Using Garmin GPS Devices for Audax
I'll start this by stating — I have only ever used Garmin Edge devices. I have never used Garmin eTrex or any of the other non-cycling or legacy units. I have some vicarious experience which I will draw on when I get time. For now, Edge:
The Garmin Edge series of devices make excellent audax companions. The devices are designed to be out in the weather, they are functional and [mostly-] reliable, and the latest versions have excellent battery life, at least twice what the previous versions used to boast.
I have only ever used Garmin Edge — first an 800 in 2013. This blew up when charging in the car, due to a faulty USB charger (giving out 12V instead of 5V!). I replaced this with a reconditioned unit — see below for Garmin's excellent, albeit not-cheap, out-of-warranty service.
My latest 1030 gives me about 19 hours of runtime from full — I have had it for nearly two years and it's still within an hour of the stated 20 hours, so I'm going to take that as a true indicator. My old 800 has a battery life of about 8 hours. The long 19-hour life means I can charge it from the dynamo all day and have enough juice for it to run all night, even in winter, before switching the lights off and putting it back on charge — a real game-changer feature for me.
Notable Issues with Garmin Edge Devices
Garmin Edge devices are not perfect. However, the issues are not ride-ending if you understand and work around them.
The 300km lock-up
This is the most serious bug I know about with current and previous models: after about 300-350km of recording a ride in the UK then the device locks up and requires a reset, which will probably lose your recorded track. The problem appears to be a full-buffer issue and is present because Garmin (and other GPS manufacturer's) rarely seem to test beyond the length of a club run (i.e. about 100-150km).
The workaround is simple: every now and again, and at least every 300km, Stop the track and Start it again immediately — i.e. press the Start button twice in quick succession. It appears that momentarily stopping the recording clears the buffer and you get another 300km space to record.
This isn't so serious for many riders, since auto-stop/start will do this at every control, and many riders do this manually anyway. Me, I hit Lap at each control, and I suspect (but haven't tested) that this has the same effect as Stop/Start.
Note that you don't need to save your tracklog and start a new one; just hit Start button twice and then visually check it is back in record-mode.
I have successfully recorded my efforts on both London-Edinburgh-London 1400 and Paris-Brest-Paris 1200 in a single tracklog by working around this issue, although I have experienced the issue on other, previous rides (i.e. I learned the hard way).
The Off-Course-Warning lock-up
This appears to be a new bug from Garmin: if I ride past something interesting and turn back then the Garmin gives an off-course warning. Okay, no bother. But if I then stop for a while at the interesting location, often a bench where to eat lunch, then the off-course warnings build up on each other until eventually a maximum number of on-screen messages is hit and the device panics and switches off.
Switching the device back on and immediately pressing the Start button will continue the recording of the current track with only the seconds (or minutes) when it was off lost from the tracklog.
The problem is that when the device is off then often I don't notice for a while — I don't usually watch the screen or follow the pink line, as I prefer routesheets and scenery, and so I may miss some distance off my recording before noticing and switching it back on.
The workaround is to be aware of the issue, and check regularly, especially if you stop for a while.
The full-battery lie
This is another issue that seems to plague new Edges, but not older ones: sometimes, but not always, the battery-level indicator shows 100% for all day and then suddenly goes into low-power mode when it realises it has been lying for the past 15 hours!
Just check occasionally, and, when you suspect this issue, charge your GPS when you think it needs charging — i.e. don't wait for it to tell you, that might be too late.
Garmin's out-of-warranty service, and "Trade In To Trade Up" programme
All electronic and mechanical devices will break at some point, one day, it's all-but guaranteed to happen. When the inevitable happens, what options that are available to you can be important.
If your Garmin dies and it's out of warranty, Garmin Europe are very helpful. Call their support team and have a chat — if your device isn't totally destroyed (i.e. it is still basically in one piece) then Garmin will offer to sell you a refurbished unit at a fraction of the price of buying a new one — a large fraction, but still not full price. The only caveat is that you must send them your broken unit, which will be refurb'ed and offered to someone else.
Now, if your broken Garmin isn't too ancient, if you mention "what about upgrading to a newer model?" on the call, then the call-handler may offer you to "trade in to trade up" — i.e. return your can-be-repaired unit and you can have something like 30% off a complete order for New Stuff. My old 1000 was complete toast (filled with water on an eventful ride to Brighton after the power button broke and left a hole in the case) and I sent it back in exchange for 30% off a new 1030-with-sensors bundle. That was a sizeable saving on retail price!