( See also Jo Wood's GPS/Audax FAQ - excellent web page )
I've used a Garmin Etrex Legend for some time, on audaxes on a bike with a lot of success. The difference between this use and general use is that I feed in the route and am able to follow route without reference, or very little, to the route instructions.
The main points in choosing and using one seem to be....
1/ Must have more than basic co-ordinates, anything less than a 'show on a map where you are' is very restrictive.
2/ Don't believe manufacturers battery predictions - halve them at least.
3/ Mount on handlebars, if carried in pocket you WILL drop it sooner or later.
4/ Be willing to accept a couple of hours as preparation time for the average route sheet.
5/ Forget OS standard mapping, I have got it on a Handheld (Acer with Memory Map mapping) but restricted battery time makes it impractical for route finding. For walking or reference use though it is fantastic, holding third of England and Wales on OS 1/50000 in full colour, zoom, search etc.
6/ If you are using at night rig up a separate light permanently set on it, internal batteries cannot handle lighting.
7/ Use a fixed offset and stick to it for junctions. To explain, instead of doing turns exactly on junctions, set the turn for say 100mtr short. Then when you get to about 50mtr from junction your ARROW or line on map will point towards the next junction, so will 'tell' you which way to turn at the junction that is 50mtr ahead. The other feature you need to have visible all the time is the distance to next turn. This is so accurate you can say "turn left in 200 yds just after sharp right hander" and as you go round the bend the junction is there as predicted. Still delights me!
8/ I use the Garmin Roads and Recreation mapping bought extra. It is down to street level in a 'stick road' format and names every street in UK but there is no none road detail. It is very memory efficient with, say, all of Wales and the Marches areas loadable into the Legend in one go.
9/ I have combined serial and vehicle cigar lighter lead to load data with, I got it pre USB but I wouldn't consider anything less than USB (or Bluetooth perhaps) now, especially as my laptop is my first computer that has no serial port (can use a converter but seems long winded now)
10/ Always carry route sheet! And 'know' where you are on it (make notes?) in case of GPS failure.
11/ Garmin Etrex handlebar mount is excellent, prior to that being available I used RAM mount (really for motor bikes?) which is bulky but secure.
12/ ALWAYS connect GPS with a cord to bike in case mount fails or comes loose to save the unit.
13/ Be willing to be followed by half the peleton as using GPS is far faster than route sheet.
14/ Must have a decent mapping on computer or hardcopy (does anyone still use hardcopy maps at home?) when programming in the route to sort out any queries. If not sure, jump ahead and then 'work back' to find an obscure junction.
15/ TO save time, if you 'know' the starts or finishes of routes (such as the 'standard' Marple approaches) then don't put those in but 'start' at a well known feature perhaps 15 miles in.
16/ TRUST IT. If it says it is 250 mtrs to a junction, then it is NOT 175 mtrs. They are accurate enough to show (in the tracking memory) whether I put my bike in shed 1 or shed 2 after a ride! Since I started using one, the Americans have increased the accuracy (less cold war risk or some such) so my unit regularly tells me the accuracy is 5 meters or less but it seems much better than that.
The Garmin Etrex Legend (NOT the Legend C which I haven't checked out because I would probably end up buying one!) is fit for use with only a few real drawbacks.
a/ Battery life (2x AA) is about 8-9 hours so is OK for most audaxes (especially if switched off for known starts)
b/ I need glasses to see on bars now really, still got some 'ready made' bifocal sunglasses but no good in winter!)
c/ Need to remove at café stops
d/ Is waterproof but I had to take mine apart this year to refix a multiple connector (bendy strip that is jammed into a clamp) that powered the screen. Fiddly but now works all the time and doesn't fade and go off occasionally, had to 'bang' to get it work again.
e/ Cannot get away with the 'laminate a route sheet 20 minutes before leaving in the morning' that is my instinctive way of doing things.
f/ Useless if you are not a 'computer nerd'. A non-nerd will only be able to read a Grid Ref and then see where that is on a map - then try to match that to the route. Not at all easy. Far better, in that case, to just use a route sheet.
g/ If you do an Audax each week, you WILL stop doing the routes sooner or later. If you do it once a month or perhaps every two weeks then you will be OK. SO save it for complicated routes or for ones in strange areas. h/ GPS units do not work under tree cover or in a building unless it has a 'view' of the sky.
All this is being written at 5 am because my leg (broken) is killing me and I can't sleep - which also means that I am not in the best of states for logical thoughts and writings. Not that it would be any better at any other time........
Oh! PS. If there is an accident, the ability to be able to give a perfect grid reference to the emergency services (especially to an air ambulance) may make the difference between a happy outcome or a fiasco. Next time you are on an Audax in the sticks, imagine having to give directions to emergency services to an accident - when you don't know where you are but you could only direct them turn by turn for 30 miles from a village hall .......
Hope this helps someone
Claire
PPS Did an attempt on Lands End John O'Groats (Well JOG-LE getting as far as near Exeter) using a GPS and no maps or other instructions. Got lost in Wigan (my home town!) 'cos I didn't dial in the route 'cos I could remember the route from when I lived there 35 years ago........... :)