As a fiction writer I can say the experience is much the same, though there's the additional thing where you get really, really tired of your own voice. Or at least I do. Does that happen in non-fic?
I don't get tired of my own voice, because much of the text is informational. One of my long-term goals as a technical writer is to include more of my voice in my technical writing. That's the kind of technical book I've enjoyed the most over the years - writing that is clearly informative, but also conveys the author's personal experience with the subject. I hope my book does well enough to justify a second edition, and I'll revise the book to have a little more voice where appropriate.
One of the hardest parts for me is when the deadlines are pressing enough that the revision process feels like work. If I can go totally at my own pace, I just enjoy the entire process. But sometimes I have to push and write even when I'd rather do other things. Even then, though, the process is really satisfying. I want people to know how to program because it gives them some power. Basic competence in programming takes away the sense that what we're doing is "magic", but leaves people with a sense of joy at taking on hard challenges and making something that works.
I might go back to some non-technical writing at some point as well. Writing a 200-page non technical book sounds pretty appealing after working through a 500-page technical book! Might be a nice sense of balance to do both kinds of writing, in the long run.
YMMV, but if I get tired of my own voice I find I'm usually doing something wrong. I re-wrote my first novel (http://www.amazon.com/Darwins-Theorem-TJ-Radcliffe-ebook/dp/...) dozens of times, and likely read it over a hundred, and I could tell what passages needed work by how I reacted to it when re-reading. If it felt stale, if my eyes glazed over, if I wanted to skim ahead to find where something interesting happened, then the reader would likely to do the same.
One my first readers commented that she found the final version of the book vastly more readable than the earliest one she read, precisely because the voice was livelier and more varied. In the earlier draft she found it "sounded" too much like me all the time, and while my natural voice is not totally boring, too much of any one thing gets dull after a while. Readers invest a lot in our writing, and deserve to rewarded for it in as many ways as possible, from the ideas and characters we show them to the pleasure of the words flowing through their brain.
In non-fiction I've not found this to be such a big problem, although I'm looking less for artistic effect there and much more for clarity, although I don't think I've written anything over 100K words in non-fiction (plenty of things in the mid-10K range, though.)
Yeah. I've written a few non-fiction humor books that don't make me laugh but others enjoy. It's similar to how I can't tickle myself. The only way I can enjoy my writing is if I forgot that I wrote it.
