This topic comes up so so much maybe there should be a sticky. If you search you will find numerous threads on this.
I did not buy one, and people seem to get their ******* in a bunch when I post information that shows it's a waste of money. Too many are brainwashed into the "peace of mind" BS every dealer peddles. This is where they make money!
If you plan on mods then it shouldn't even be a consideration IMO. I'm running out of OEM parts..
$1400 for 7 years and 100k miles isn't that bad. Replacing suspension on these trucks is going to cost at least $1400 from RPG or Outlaw (and probably more if you got the stock suspension from Ford) and that doesn't include labor. If you don't have the mechanical ability or, more importantly, time to repair your truck, the extended factory warranty isn't a bad option.
Like you said, it depends on how much you modify the vehicle also. If it's heavily modded, probably not a good idea. Lightly modded (mid perch, wheels, tires, lights, etc...) it's something to consider. If the MFT or SYNC goes out and you want to retain stock look and ability you would pay out the nose to get it replaced from Ford.
If I were buying a Toyota Avalon, there is no way a dealer could get me to get the extended warranty. With a Raptor, I'll shop around on the internet and find a good deal for a 6 or 7 year 100k warranty which is going to cover everything if and when I need it to.