It can be used for sleep tracking just fine and for me is the reason I only put it on charge in the mornings rather than over night. I find that sleep tracking in general isn't great for me though, as I tend to struggle with sleep and lay awake a lot without moving much. Any watch I've tried will put that down as sleep more often than not.
Since brexit there may be customs charges taxes etc
Don't expect accurate GPS tracking results. Even on highest setting it takes too few points to follow your route, particularly round bends. On a track loop you can expect it to read 5% below actual distance travelled. Other than that a good watch and easy to load your own music files (from a Samung phone anyway - not sure about others).
So actually it can't be used to monitor sleep which after activity monitoring is by far the most interesting and useful feature of smart monitors. 2 days charge is a non starter, not sure how they can compete with most alternatives offering one week or more
I purchased S3 watch from Spanish Amazon and there was no way to make Samsung Pay work in the UK (confirmed by Samsung UK).