A wall of text incoming:
Imagine it like this: FM synthesis is a very deep ocean, like thousands of meters deep. Now there’s always people who are interested in deep oceans.
And there’s certainly interesting shit down there that can’t be seen anywhere else.
But is it still necessarily more enjoyable than living on the shore, enjoying the sun and drinking mojitos, diving just a little bit to see some coral reefs?
There’s definitely a reason why FM8 is so popular, especially among more experienced artists who know it inside out, but it’s not like every Massive or Serum user for example would enjoy it. Same way as not every diver is interested to grab a submarine to see these unique things that lie in the bottom of the ocean. However some of those who do, can get really hooked into it and spend rest of their lives doing research on these alien-like animals we have there.
I would wait for a discount (NI stuff, including FM8 is quite often on sale). When it’s on something like -50%, it’s absolutely worth it no matter what. But I wouldn’t pay the full price if I had Serum tho. Because Komplete 11 costs 600€ (regular price) while the full price of FM8 is 150€. And the value of Komplete (which includes like every NI software product) is still way more than 4 x FM8, especially when you still can stick to Serum (or any other modern heavyweight synth really) for the most stuff.
EDIT: Or then try the demo if there’s one (go on and find out).