One of the issues I ran into was choice of research topics- there simply weren't any that interested me all that much. Now that I've been working for twenty years, I can think of all kinds of problems of practical significance that I'm too busy to try to solve.
Another issue to keep in mind is that in my experience, people get pigeonholed very quickly. If you have any experience in any field, then nobody even thinks about interviewing you for anything but that one field. It is very peculiar to me to pay engineers good money based on their intelligence while at the same time assuming they are too stupid to learn anything new, but that's the way the system works. With electronic jobhunting, it seems every opening must have about 800 applicants, all of whom are already working elsewhere, so it can be a frustrating mess. I think this effect would put you at a disadvantage in job hunting for "normal" jobs when you have a PhD.
Re the comment: "Every once in a while I run across advertisments for PhDs in engineering and profesional society journals. The IEEE's monthly magazine usually has a bunch."
What I remember about ads like that is they are nearly always universities looking for faculty. I don't know what the demand is for professor types right now. Either there's plenty of PhD's or there's not. If there's plenty, you've got an uphill struggle to get on to a good job that way; if there's a shortage, it'd be a bit different. When I went to college 20 years ago, there were still two or three of the old professors or associate professors that only had MSME's. But all the guys under 60 had PhD's, and even that only seemed to get you on as a "lecturer" if at all. Professors seem to make good money, but getting to be full "professor" is a bit of a trick, it seems.
I remember that I used to see these weird ads in the ME magazine. They'd go on a for long paragraph about the exact technical qualifications required, experience with this or that software and this or that application, and then you'd get to the end, and the pay would be substandard. I finally figured out those weren't ads looking for anyone- they were ads for positions currently filled by foreignors, with the intent of showing there were no "qualified" applicants and that the existing job holder's visa should be extended. Seems a peculiar way to work things.