I actually haven't bought one yet because I was under the impression code rendering was still less than ideal (e.g., lines wrapping to fit the Kindle screen where they shouldn't in code). Is that still an issue?

Yes, this is an issue on the Kindle device, but reading textbooks in the Kindle app on the iPad is gamechanging. The speed with which you make highlights, leave notes, and bookmark is great for learning. I can then access the book from my computer for quick reference and/or convert my booknotes to Anki flashcards.

This being said, I also own a Kindle Paperwhite and use it at least an hour a day for reading non-technical books. I can't recommend that device enough.

Would you recommend people who already own a tablet (iPad, Galaxy, Fire, etc) to get a dedicated device to reading?

I have both a tablet and a kindle paperwhite. I like using the tablet for technical references which are commonly in PDF format. For everything else I use the Paperwhite. The backlight is much easier on the eyes, especially when reading in bed.