We have an '04 OBW that we bought new, and I've over-maintenanced to death. Last September I thought I smelled antifreeze while driving slowly through a resort community. Yeah, the overflow bottle was suddenly almost empty. Log onto subaruoutback.org, and it was like Welcome to Headgasket Hell! I took it to the dealer, and they suggested waiting till timing belt time to fix it. Gave me a little bottle of Subaru coolant additive, which is re-branded Holt's Rad Weld, and sent me on my way. It seems there is one major flaw in a horizontally-opposed engine, at least in Subie's version. After shutting the engine off, hot coolant sits at the lower back end, and the coolant, still under pressure, weeps through the gasket. According to what I've read, it almost never weeps into the oil or combustion chamber, but just to the outside. The danger here is losing the coolant and overheating, something that oddly shaped aluminum castings cannot tolerate well.
It's a shame to have such a major design flaw, because up till now, it's been an amazing little beast, and we've been to many parts of the country in it, all the while averaging about 32 MPG driving with a sensible foot.