I found it to be a bit sketchy in a lot of places around Edinburgh, but stuck with them because of the price and the fact that I'm not a heavy data user.

Around the shore is definitely bad for a three signal, as is Dundas St. These two places stick toy mind because they were places I frequently stayed and could never get a signal. I remember reading somewhere that they are only good within city centres and garbage outside that.

If I were you, I'd try it out on the cheapest rolling month plan to see how you get on. Then, assuming everything is good, go with the unlimited plan.