If you want to be legal, it has to be GPS based. I run a Talex and it's excellent:
It's £150 and you need a subscription after (IIRC) the first 6 months. The alternative to this one is the Micro Fuzion:
I reckon this is built by the same people as the Talex. £99 and no subscription. This has just come out and they reckon they are the cheapest. My colleague is using one and thinks it's great - I've just bought one for my father-in-law for Christmas...