Is heavy vinyl worth it?

The quality of the vinyl is usually better which results in lower surface noise and less pops and clicks, but it only really makes sense to consistently go 180gm and greater if you're willing to adjust your VTA (and other settings) on your tonearm.

I guess the answer must be, "it depends". For us the 180/200g issues are definitely worth it. At a minimum, classical reissues on heavy vinyl tend to have quieter surfaces and less warpage.

Heavy vinyl usually accumulates much more statics than regular. In case with 150g you can get away without VTA adjustments while 180...200g will likely need it.

I set my TT up using heavy/thick LP stock and add a second "slim" wool felt mat when playing thinner LP's.

I do not hear much of a difference between thick VS thin LP's as long as both are flat and in good condition. My TT is located outside the listening room, I use a non-clamping LP weight and I do not listen @ high volume levels (which may have something to do with this).

The only big step "up" for me (amongst recordings that are decent to start with) are the 45 RPM versions.

Complicated. The old problems were crappy vinyl, thin pressings and uncontrolled cooling or curing. The new problem is, can a re-issue be done to at least approximate the old stock (or even do much better, like fixing the wrong speed). I have late 70s early 80s that are so thin the sound bleeds from the other side. No wonder the CD people found ready targets. Most new is on 180-200, which can affect your tip attitude and sound less then it is. You may need to lift your arm a tad to lock back in. Course that is a problem for your regular LPs. My last comment is that it is the maker more than the thickness.