I've used LyX for a very long time. It has the best graphical equation editor I've ever used: it natively supports all of the complex structures you'd want, can be used incredibly efficiently via the keyboard (e.g. tab-completion and tab-navigation), and is still incredibly discoverable via GUI.
In general, it's just a very pragmatic layer on top of LaTeX. I've done a lot of complex ad-hoc formatting in it as well.
I like, that one can define macros in LyX. For example I wrote a simple macro that looks like "paren(thing)" which is then translated to "\left(" thing "\right)". This makes it much easier to write formulas, because I don't have to keep track of parens at all. LyX in this way makes it more convenient to write TeX/LaTeX.
In general, it's just a very pragmatic layer on top of LaTeX. I've done a lot of complex ad-hoc formatting in it as well.