I get the GBG every year as a CAMRA member and I have to agree with B R Drinker "S.A.D." to a certain extent. There is also the problem that some places don't get visited very often by whoever chooses which pubs go in the Guide for that area whereas other do (look at the number of entries for St Albans!). Plus some pubs in the past have been marked down and taken out of the Guide for reasons which escape me -- I suspect that "poor beer quality" sometimes means "I didn't like the look of the other drinkers".
Some pubs will never get in the guide (McMullens managed pubs spring to mind as they use cask breathers) but the absence of an entry does not automatically mean a pub is poor.
Having said all that, it is invaluable in those parts of the country with reputations as "beer deserts" and doesn't just list the pubs but also the independent breweries, so I won't knock more than one star off.