We've had our X5 for less than a year, but I'll give it a go.

- I knew that the 4.8 was prone to oil leaks. Which I unfortunately brushed off as I've dealt with oil leaks on my E36. I really wish I previously understood the amount of labor it requires to get to what is leaking oil. We had the oil t-stat gasket go out and I can't remember exactly but it was 8-10 hours of labor for a $10 gasket. On the other hand I was able to replace the vacuum pump O-rings in about 30 minutes which saved a chunk of change since the shop wanted to replace the whole pump with an OEM unit. It probably wouldn't have really scared me off having the knowledge prior, but I definitely would've looked to buy one that had them done recently.

- From what I know (4.8): valley pan/coolant pipe, valve cover gaskets, timing cover gaskets, oil t-stat housing gasket, rear air bags, coolant expansion tank, sunroof can bind up.

- We're at about 10 months of owning a 2008 4.8is with 138,000 miles and we've replaced the:
battery (dealership because of coding) - $300
air bags (DIY) - $380
oil t-stat housing gasket (indy shop) - $1300
vacuum pump O-rings (DIY) - $15

I really don't see that amount of repairs year in and out. I will probably DIY everything now since we're both closer to work and I have more time, plus we have a back-up car. I don't regret paying a shop to do the oil t-stat gasket even with so much of the cost of it being labor. It was January and it sounds like a pain.