1. Does anyone have any experience with any of these publications that has been either positive or negative?

2. Are there other potential mainstream magazines that I should also consider?

3. Is there a good place to find lists of such magazines and/or their submission policies?

4. For an unpublished author, is there a better way to proceed than just sending a first story in, carefully following submission guidelines? I do have one published book of a technical/nonfiction nature. Does that help at all? And if so, how can I take advantage of that?

Check out www.ralan.com or "Novel and Short Story markets" -- a book.

There's a lot of other markets out there beyond the Big Four (the three you mentioned plus Realms of Fantasy), and many of them may be far more fitting for your submission depending on what it's like. Two I've heard good things about are Strange Horizons (if I have to be rejected, theirs are lovely) and the Writers of the Future contest.

In general people say the properly formatted (and some pubs, especially online ones, will have their own format guidelines listed) short story with or without cover letter probably is more impressive than anything else. If you have a nonfiction writing credit, you could put it in your cover letter, but a lot of editors say they don't read cover letters, and many of them say they only care about non-story *anything* where it relates directly to the story (eg, a "Stars are Sentient Goldfish" story will only fly if you've had a paper on the topic in Journal of Star Formation recently and can defend the science. Sentient goldfish borrowed from Myrtle here).