I've got a 2006 2.5l NA Forester, with a 5 spd with just under 190,000km on the clock. It's my second Subaru (Had a 1997 Impreza before that).
I bought the Forester used a couple of years ago, complete with the HG oil leak. Being aware of the leak made it easy to bargain for a lower price. I kept an eye on the leak and the coolant temps, and when the mileage rolled into range to do the timing belt and water pump, I had the HG done at the same time to save on costs for pulling the engine, etc. Since then, it's all be tight and dry. I've known people to go years with leaking HG without having any major problems. However, on my old red car, I had a HG fail internally within a matter of days, sidelining the car. I'd rather get them replaced than end up stranded somewhere. Even with all that said, they're great cars.