Which model years where problem and which year did they become ok? I've been looking at expeditions and the like but when I googled I saw many sparkplug spitting complaints
I have a 2005 Expedition and it has been a wonderful truck. With 300 hp, it performs quite well. The IRS is very comfortable. I do what I consider to be a fair amount of towing, especially in the summer months, and the truck performs admireably.
I have 75,000 miles and haven't had many problems. I'm about to replace the 45k mile old tires with new rims/tires and giving the truck a 'second wind' with new ball joints, belts, plugs, etc. There has been very minor wear on the driver's side leather and the steering wheel has shown some wear. Other than that, the truck is brand new.
I did have an extended warranty service of the 4WD system...the switch from 2WD to 4WD became 'reluctant'. But all in all, it has absolutely blown away my expectations of any truck.
I normally get 14 to 15 MPG in pure city driving, 18 to 20 MPG on the interstate, and a solid 11 MPG when towing my 5,000# bumperpull travel trailer. A diesel would always be nice, but the 300 hp 3-valve engine is very, very nice. I mirror Chris' suggestion and restrict your searches to that engine, up from the 2004's 260 hp. My parents have an earlier Expedition (240 hp and solid rear axle) and it is much less pleasant. Absolutely no comparison to the 2005 and later trucks.
Ultimately, mine will be replaced with a larger P/U, probably diesel, simply to accomodate a small (under 30') 5th wheel...the next logical step in our outdoor life with the kids.
But unlike other cars I've owned, I don't look forward to replacing the Expedition. It's been great!