Here's my 2 cent crystal ball prediction: If you are a gamer, no matter how you cut it, Zen will bring benefits, even in the 4 and 6 core variants, for the simple fact that single core performance will improve and many games run on less than 6 cores. In multithreaded performance, 16 threads will help only people that run very specific applications that actually can see a benefit from HT. In Intel, an article i had read, was saying that HT was bringing at most a 20% benefit, in a handful of applications, that could saturate all cores and use HT at the same time. In some other applications like games, HT was even deemed negative, which led gamers to often disable HT to play some games.
But, i will say this. The FX isn't dead yet. It won't be dead for many years for general use and won't be dead for several years for games either, since more and more games tend to use many cores, finally.
I also think, that no matter how good Zen will be in the immediate future, since it's completely new design for both CPU and motherboard, the real star will be Zen+, because by then AMD will have fine tuned everything and most likely improved clocks too. Personally, i wouldn't invest long term in Zen (buying top gear), since i am pretty sure that when Zen+ comes out, the "old" Zen will look like Bulldozer compared to Vishera or Phenom I compared to Phenom II.
I think for gamers, the smartest thing from price/performance point of view, will be the six core Zen with a 100eur motherboard and later go full blast Zen+ and new gen mobo (i expect that there will be some serious revision of the motherboards when Zen+ will come out and everyone will want the new, bug free and stable AM4+ motherboards).
If you aren't a gamer and simply use your PC for generic purposes, which as you say, never use your current 8 cores, you may as well sit as you are and wait for Zen+.
EDIT: And now, instead of reading predictions, you can wait 1 month and learn for sure, from the first Zen reviews.