In answer to the question... TM Subscription was £30 last year and I find it both incredibly useful and very annoying!
They usefulness, as TB states, does rely on your intelligence too. Sometimes it'll tell you of a delay but it's just not worth the hassle of going round it! I find the best usage is to combine TM, a BBC Radio station with TP turned on (normally playing CDs instead of the Radio anyway when on a long run), and a telephone service, like 401100, combined give the best picture of what is happening

It's intensely annoying because it only give information for the next 10 miles, it only covers the Major road network (with blue camera's at the side of the road) and sometimes a transmission will get stuck and you'll get reported delays which aren't there. I had this a few weeks ago for 5 days, at all times of day and night!
In summary, I would say if you travel enough miles you will probably benefit from it... I know I have and I'm planning to renew it when it runs out
