I tend to stay well clear of those that advertise on the basis that if working on the premise that those without work need to advertise the most, by implication those that advertise the least (or indeed not at all) have the most work and by virtue must do so because they really are quite good.
Although in general i do agree that word of mouth is a great way to employ i have come across many issues (my sister included) were they got screwed by an incompetant builder to the extent i had to drive 150 miles to correct some god awful plumbing, the comment above i feel is a little short sighted, for example British Gas advertise loads and have loads of business, Pimlico Plumbers advertise loads and have loads of business, Dyno Rod advertise loads and have loads of business, For me (or any plumber) to grow a successful business this could not be done by word of mouth alone. I advertise mainly on Google and Thompson Local but i do get much business from word of mouth.