I don't use it. I think it might be useful for a user that is not familiar with the database. I would use auto-complete more if it would complete column names, but it only seems to complete schemas/tables/views. So in the end, I just type it all in.
I'm wondering how many people is actually using Graphical Query Builder.
Coming from MSSQL (last version used was MSSQL2000), with it's Enterprise Manager and Query Builder... I never used it. I always found it much easier/faster to type the query in Query Analyzer - as I'm doing today, using the SQL window in PGAdmin or typing query in pure psql.
But, I guess that is just me... that is why I am wondering how many people is actually using such things as Graphical Query Builders?