My experience: On a LDP I use (printed out) OS maps, because they show what is around you. I certainly would not use the GPSr and no maps. That way you lose half the fun and interest.
Some walkers use their GPSrs instead of maps. I navigate by map as much as I can because I like to know where I am relative to the landmarks around me. I use my GPSr as a backup rather than as my primary navigation tool. In difficult circumstances, such as heavy rain at night, or freezing weather, I will use the GPSr to find my way. So battery life is not a problem on a multiday hike because I will only use the GPSr once or twice.
Normally I plot my route on the 1:25K mapping software I use (Tracklogs), upload the route to my GPSR, then print out the maps with the route shown on them on A4 sheets using a colour inkjet printer. The mapping is at 1:25K, but I usually print out at a 1:20k scale because I find this easier to read. If I need to read the maps at night (eg on a 50 mile challenge walk) I print them out to a 1:15k scale. For a LDP I would print out a strip of 1:25K A4 maps at 1:25K or 1:20K, covering the whole route
Having owned and tried several Garmin GPSrs and tried a Satmap, I much prefer Garmin. Others prefer Satmap. If you already have a Satmap, it?s not really worth changing it for anything else. The two basic functions are to be able to record a route while you are walking it, and to plot a route on mapping software, then upload it on the GPSr. All advanced GPSrs will allow you to do this. All hiking GPSrs will be waterproof to a high standard. Weigh varies, but they are all quite light. Satmap screens are bigger than most Garmin screens. Battery life varies also, from about 20 hours down to 10. On a one day hike this does not really matter, but on a multiday LDP you will need to recharge batteries or buy new ones along the way, if you use the GPSr all the time. The Satmap and most Garmins use AA batteries, so are widely available.
Slow satellite acquisition was a problem on the early Satmaps, and the makers produced a software fix. So download the latest software and see if that makes a difference. If it doesn?t, take it in to the shop. Satmap are reputed to have excellent customer service.
On mapping software, the first thing to be aware of is that while you can plot a route on your mapping software and upload it to the GPSr, you cannot upload the maps themselves. You can buy 1:50k and 1:25k OS map cards for the Satmap. Your uploaded route will then overlay the map on the Satmap. One aspect of the OS map cards I don?t like is that they are raster maps. This means that when you zoom in, everything increases in size, including labels and icons. On the small GPSr screen, this means your screen soon gets filled with enlarged, irrelevant information. Other mapping for the GPSr is vector mapping. Here the mapping is in a series of layers, labels (eg street names) on one layer, contours on another, roads on another. When you zoom in, some layers, eg labels, do not increase in size and hence do not obscure everything else. But the vector maps I have seen do not contain the detail that the OS raster maps have. I find the non-OS vector maps better and adequate. You can get free UK vector mapping from Talky Toaster for the Garmin GPSrs. I find these better than the Garmin vector maps. If your Satmap came with OS 1:50K as they often do, I wouldn?t bother changing it for anything else. If you walk in particular area a lot, you find it getting an OS 1:25k tile of that area. Check my information on OS raster maps. If they offer OS vector maps, that would be much better (but I would still use paper maps rather than the GPSr ? much more fun and informative)
For Garmin GPSrs, not Satmap as far as I know:
http://talkytoaster.co.uk/ukmaps.htm
If anyone uses these, download the maps in the morning (servers not so busy), and follow the instructions, especially about renaming the file, carefully.
OSGB allow access to their 1:50k and 1:25k mapping online for £20 annually. You can thus plot your route and upload it to your GPSr, and print out the maps also. I haven?t used it myself, but others say it works well.
http://www.magazine.ordnancesurveyleisure.co.uk/magazine/tscontent/general/getamap-subscriptions.html
https://www.ordnancesurvey.co.uk/opendatadownload/products.html
Finally, a GPSr is a great toy and can be a useful tool, but the real MacKay is the map and compass.