National Book Tokens can be used in any participating shop, but only for books. As far as I'm aware in the UK The Works doesn't take them, but any independent book shop will and it's a great way to support them. The only reason I wouldn't buy them is if they were for youngish children who might really want toys or stationery - which most bookshops also sell because disappointing small children isn't fun for us.
If you have a good independent local bookshop, or know that your intended recipient has, that does its own gift vouchers it's a great way to support them and whilst I work for a national chain I strongly believe we all do better when there's more choice and good local competition - or perhaps more accurately, the independent shops around us can do things we can't which means a richer literary scene for everyone if we support them. The local bookshop scene across the English and Scottish borders is fabulous, and a testament to what supportive communities will get.
The advantage of buying a voucher for a big chain are that most people will be reasonably close to a branch in Mainland UK and you get a wide range of games, jigsaws and other bits to choose from if you're not entirely sure books are what will be wanted.
I like getting tokens because it's something to open, and then there's the anticipation of what I'll buy, followed by my final choice - it's nice to have something that can only be spent on a self-indulgence with no thoughts of saving or using it for a sensible cause (my front door apparently doesn't meet current fire regulations and will need work done to bring it up to code in the new year).
I like giving gift tokens because I can direct the spending of whoever is getting it - normally the children of friends who have given me instructions - whilst still providing the excitement of getting a choice. Or because I know I'm passing on the same pleasure I get from them.