Some guys on the Ford forum are starting to see a trend of cam chain (or associated component) failures on 75k+ mile 3.5 Ecos. I've got 52k miles on mine, tow heavy (8500 lbs) a good bit, and have had a recent warranty repair for blown exhaust manifold gaskets. IMHO the bolt pattern to secure manifolds is a weak design around the rear cylinder.
I have an extended warranty on mine and will be selling it at 95k miles. It's a small motor, and I stress it hard. I think it's going to be an issue over 100K. I also run a 5* tune on mine that adds a good bit of torque and HP so I'm sure that won't help longevity, but the extra power is awesome and I can't drive it now with the stock tune.
I drive normally - not fast or hard on it, except the stress of towing heavy. I get 15-17 normal use, 9-11 towing heavy. With the tune I'm around 400 HP and almost 500 torque, and it's a 2 ton brick, so no complaints on the mileage.
I'm sick of some of you morons arguing over stupid $hit too. Just the facts, ma'am.
Mike