My Dad asked the same question of me last time they visited, so I'll give the same answer here I gave him - but a little background first: been in this house 15 years (2000 sq. ft., hydronic heat main floor with in-floor in the basement), no natural gas out here so first four years it was 100% propane (have a 500 gal. tank), then in the fall of 2004 I put in an OWB to heat the house and shop and hot water; after the first season with the OWB I shut it down in the spring and let the propane heat our water but after a month of not having the smoke aroma when stepping outside to go to work and not seeing a wisp of smoke from the stack, I guess I was going through withdrawals so I lit the girl up again and it hasn't shut off since the summer of '05, except for the annual top to bottom cleaning and maintenance. With five in the house (two teens and one to be shortly) we go through a ton of hot water so we burn year round. The propane boiler serves as backup and I run it every few months just to ensure it's operational. So far so good.
So, when my Dad was last here and pointed at the OWB and asked how much propane I go through now that we've been using the OWB, I said: "each year I burn more propane in our outdoor grill than I do to heat the house".