I've had my MY05 3.0RB Wagon since 08 (9 yrs) and it's been a rock solid family car - good for commutes, longer drives and camping (and of course the occasional spirited hard driving).
Love the balance between luxury and performance. But will soon look at mods when a new bigger car is required, so mine is completely stock - can't comment too much on upgrades yet.
Planning on Raptor SC and headers + exhaust + Brembo brake upgrade when this becomes my mod car. It's that or the Focus RS
)
Here are the maintenance issues to look for:
1) I've heard of more than 3 liberty wagons with rust issues on the roof along the plastic moldings that run front-to-back. Small bubbles near the roof rack mounts (I've had mine done twice now both left and right sides to cut out, weld and restore looks as good as new) - put electronic rust protection on the car if you intend to keep it long term.
2) The shitty central locking actuators. The doors need to come apart, take out the white plastic actuators in its sealed casing and crack it open, clean it out then replace with OEM motors. (subaru want some ridiculous amounts to replace them, but I managed to get them all done myself for about $30 of motors off ebay and a few hrs work. The casing needs to be heated up to loosen the glue, cracked open with a knife and then motors replaced. Casing is resealed with silicon and stainless nuts and bolts, I doubt you'll ever need to change them again. Original ones failed because the actuators were filled with too much lubricant grease getting into the motors. There's a thread in here with a walk through. Kudos
3) Oil use between services, unfortunately mine does drink a little oil - between 500ml - 700ml every 10k, enough to notice a difference on the dipstick but not enough to worry about. Read reports that is due to improper run in, but seen enough people have it that I reckon it's part and parcel of the H6. Just watch for it and top up regularly.
4) Had my clutch replaced @ 120k, now at 220k still going but will expect it to need attention soon. Mid engine mount also replaced. Suspension bushes replaced.
5) Electrical - replaced reverse and neutral switches replaced when worn out (it's at the top of the engine under the intake - so all the top has to come off to get to it.) A couple of relays, bulbs... (Front left H7 bulb always seems to go first)
6) Electrical - reversing light - had mine traced at S-Technic and found it was a broken wire in the loom between door and pillar...they mentioned it was the second one to come in with the same issue.
7) Speakers - my OEM paper cones blew last year and replaced them myself with kicker drivers from JB HiFi direct bolt out bolt in replacements (sounds awesome)
So there it is - might seem like a big list, but that's over almost 10 years of ownership, and the car still looks pretty damn sexy.