Truth be told the 4.6is is a bit of a bear. Even mint examples suffer from depressed values, maybe due to the 4.8is? But definitely the engine can be a bit scary since if one blows a 4.6L, it's $$$ to get another.

I recall a very nice 4.6is in my area not too long ago for a very hard to pass up price given it's condition, basically mirroring that of danny's X5 sans 100k miles. (Tow pkg, LCI steering wheel, C-L-E-A-N). Poor thing ended up sitting on the market for quite a while.

Basically needs the right buyer to come along I guess