Differences among Doctoral Degrees/titles I have two colleagues who hold doctoral degrees. They are moving to the US within the next couple months. Both of their doctoral diplomas, one from Europe and the other from South America, literally say: "Doctor of Engineering". 

We were discussing about this situation and, although they are doctors, I don't know if it would be "legal" for them to use the PhD letters at the end of their names or if D.Eng would be the adequate choice. So far, they have not been asked to do any kind of nostrification process.

Are there any legal, formal or professional implications associated to this or is it just a matter of putting a set of letter at the end of your name?
An EngD is a different degree to a PhD. If you've got a PhD you can't put EngD after your name, and vice-versa. Both can give you the title "doctor", but they are different doctorates.

If your degree diploma says "Doctor of Engineering", it's very very likely that you can put Eng.D. after your name. But not PhD. (and I've never seen D.Eng after a name, only Eng.D. - but maybe that's UK-specific. I have seen Dr.-Ing. from Germany)