1. Are you glad you did it? - YES
2. Would you do it again? - YES, although I'm back with KDP for a new pen name early 2017
3. Do you see yourself moving back to KDP Select any time soon? - not with the series I have at the moment, but I'll see how the new pen name goes.
4. & 5. follows:
It took me a good 4-5 months to get a handle on the other stores, and that followed a Bookbub, but they're good for a few hundred dollars now. I never really connected with KU on the series I had in there in early 2016, so I'm happier wide.
Random thoughts as they occur to me:
For formatting, I use Caliber, which is free, and converts the exact same files from Word (.doc .docx .rtf, etc) to epub and that passes both Draft 2 Digital and Smashwords file checkers, as well as Kobo and Nook.
I use Draft 2 Digital as the aggregator for Apple & Nook, plus Scribd and the other outlets on there. Nice dashboard, easy upload process, fast price alterations. I've gone through Smashwords for some of the more niche ones but never sold a single copy there.
I go direct to Kobo because it's so easy, and after a while you can use their own advertising options.
You can go direct to Apple but you need a Mac to do so. I haven't gone directly to them yet, but I hear it's tricky the first couple of times. After that it gets easier. The big advantage of going direct is that if you're moderately successful they are likely to help you promote directly to their readers, which is another plus on top of the slightly higher royalty rate you get.
Apple won't publish your book if it links to Amazon, so you need specific links in the back. I have links to Apple, Nook, Kobo and my website, which they don't seem to mind.
You can also go directly to Nook, but I'm not sure how much benefit there is in it besides a slightly higher royalty rate. There WAS a Nook promotional tool if you're direct with them (NOOK First --
https://docs.google.com/forms/d/e/1FAIpQLScsi3zjcLDcn2mAasN5yTzSk8Jc_DhlkGJtJm7rBpinAu5vVw/viewform and NOOK Price Promos
https://docs.google.com/forms/d/e/1FAIpQLSfh5ZPyVSnRgP3npAuddtYFEq2PFpj_NjWz3B11EvjFFPDBhA/viewform), but I never saw a big increase through them, at least nothing that stuck. I pulled out of Nook direct and went through D2D because at the time you couldn't do permafree through them. I understand that's changed now, but you'd have to check. Again, slightly higher royalty rate, but it's another channel to keep control of.
Ultimately, you need a lot of patience. I suspect a really hot mailing list helps, although I'm still in the low hundreds. A decent marketing plan, and if you get a Bookbub early on I'm sure that will accellerate the success or otherwise.
Best of luck if you take the plunge.