Right! The Garmin - It's taken me a couple of weeks to use this properly. On getting it I found that Garmin (the company) have stopped producing items specifically for it, and more or less discontinued it in favour of their Garmin Edge 800. The one I have - the Garmin 705 - is more or less a 2008 model (the equivalent of getting a plain old Ipad (or ipad - as opposed to an I (or i) pad 2 - had to make that distinction, the Giddy Blondes back-seat writing) and about £200 quid cheaper.
The BaseCamp map that you get with it has only really motorways highlighted - and I couldn't find any definitive screen shots to ascertain this, so you need to buy a memory card with the maps already on it (from Garmin) or download these from another source (I used this guy here:http://talkytoaster.info/ukmaps.htm - this is primarily for the UK and shows 1 50k ratio).
Uploading a route is a bit of a faff and is easiest using Firefox.
We needed the file downloaded from here: http://www.elsewhere.org/journal/gmaptogpx/ in our bookmarks on Firefox first.
The way we did it (the Giddy Blonde worked it out actually) was that we made up our routes using Google Maps, then went to the gmaptogpx option we already saved in bookmarks, then told us (well, me) there is an error and we needed to download it as a KML file (which kind of looks like an old fashioned HTML file but don't fiddle with it). We did this and then convert to GPX from this programme here; http://www.gpsvisualizer.com/convert_input (this link a appears in the error message).
We then copied (Ctrl A) all and pasted it into a notepad saving it as 'whatever.gpx' - it's the' .gpx' which is important - the way we do it, it always goes into the garmin as 'Course.gpx'. We then had to save this as course.gpx onto the memory
Anyhoo - we managed to get the 50 miler onto the Garmin. And on that side, I'm very happy. With the exception of one or two wrong turns (my fault - I had 2 legs of the trail go up a little road and back down again), it worked. It beeps about 20 seconds before the turn we had to make, then showed the next one coming up straight after with the distance of how far away it was. The screen was big enough to see that one of the wrong turns we made - we didn't have to u-turn - and once we were back on the train the Garmin picked us straight back up again.
The cycle computer side, was a bit fiddlely . I've set up a few bike computers now, so I expected this to be fairly intuitive, but when we were about half way round the test route it showed that we'd only done 7 miles. Now, either I hadn't set it up right, or I was looking at the thing wrong but after fiddling with the wireless sensor, (short story - taking off the pedal sensor, resetting the GSC 10 sensor so it only picks up the sensor on the wheel, checking the wheel size was right - don't use the auto sensor it resets everything to 2100mm which doesn't work for 700x23mm tires - and then putting the pedal cadence sensor on) I think I've got it working.
Here's the ridewithgps.com results for that first ride - see how it says 16.5 miles?!
A tip is that to hook it up to the computer actually then switch the device on, it doesn't act like a memory stick, and even though there's a recharge sign on the Garmin, it's not in 'Talking to your computer mode'.
Would I recommend getting one? YES. I'd take it out on a shorter first ride to check things were working as they should. The routing it does really was rather good and I trusted it as there was a couple of points where I only had a rough idea where I was.