Having recently bought an eTrex 30 I was keen to get obtain some mapping to use its full potential. The new Garmin GPS units allow you to download 1:25K OS maps at a cost of £20 for a user defined area of 600km2. This is actually a very small area making the Birdseye Mapping extremely expensive. The Birdseye maps are also very poor quality.
The Garmin GB Discoverer maps are 1:50K scale, but I find that this is perfectly adequate for walking with a GPS. The GPS tells you where you are so the level of detail is less important compared with using a paper map and compass. Garmin sell this product for £200 on their website, but it is available for £150 on Amazon making it good value compared with other options.
In my opinion, buying the full GB mapping will make me more likely to use my GPS as I have made a one off investment that I might as well use. The alternative of downloading Birdseye maps at £20 for a small area, would make me think twice about whether I needed the map and I probably wouldn't bother for small walks. For example, if I went to Scotland for a week and planned on doing 4 walks, I would be faced with a bill of £80. GB Discoverer is good value in comparison.
The main downside to the product is the fact that is stored on a micro SD card which can not be backed up on a spare card or PC. I you lose your GPS or wipe the card the data is lost. Personally, I would prefer to be able to store the data on my PC and just transfer the relevant data to the GPS when I need it, but this option is not available.
Overall, I am happy with my purchase.