3900x looks fantastic on paper. In general, if the Ryzen stuff is sufficient for your needs, it's a better value. You pay a big premium for the Threadripper boards (and big case and big cooling solution). So in that sense, the 3900x is definitely in a sweet spot at the top of the Ryzen range.
Tradeoffs: threadripper boards officially support ECC; Ryzen boards are hit or miss. TR boards tend to be priced around $300 whereas you can get a Ryzen board for $100ish. TR had (prior generations) twice the DRAM channels and way more PCIe lanes than Ryzen, so if you're doing GPU-intense work or something else with use for lots of PCIe, that's a plus. Not to mention, additional core count over Ryzen, although with greater inter-die latency. Not sure what that will look like with TR3.
Is 3900X worth $500 at list over $400 3800X at list? Actually, yeah, it looks at least 25% better to me (esp. the doubled L3) if you can use the cores. The 3800X is overpriced; they probably are learning from the 1700<->1800 dynamic in gen1. Is it worth it over the 3700X at $330? Maybe not.
For me the question is really, how long will Ryzen 3000 be on the market before those better IPC/clocks/core densities show up in TR3? PCIe 4.0 support is huge; AMD wasn't anemic on PCIe channels on Zen and Zen+, and PCIe 4.0 doubles bandwidth from 3.0. Hopefully those IPC gains do not come attached to Spectre/Meltdown-like vulnerabilities. I'm excited for Zen 3 TR! That might be worth an upgrade from the 1950X. Meanwhile, it doesn't seem like Intel will get to PCIe 4 until 2020 (although that's reasonably soon).
I think the 3900x is in a great position to provide the best of both gaming and productivity. Extremely aggressively priced at $500 for the horsepower it seems to give you.
I suspect there is going to be a 16 core 3950x later in the year. Maybe with slightly lower single core frequencies. But maybe 20-25% greater multicore performance.
I bet they are delaying that to keep something up their sleeves when Intel responds. And to not totally cannibalize TR prior to releasing TR3.
The 570 boards are going to be around $100-200 more expensive though. The PCB is a bit different and the specifications are more tight for PCIe 4. I think the cheapest board you'll see soon will be above $150 at the very low end all the way up to $600 or so. Many of the prior two gens will have issues running the newer CPUs and the board vendors are recommending against Zen2 on chipset boards prior to 570
Tradeoffs: threadripper boards officially support ECC; Ryzen boards are hit or miss. TR boards tend to be priced around $300 whereas you can get a Ryzen board for $100ish. TR had (prior generations) twice the DRAM channels and way more PCIe lanes than Ryzen, so if you're doing GPU-intense work or something else with use for lots of PCIe, that's a plus. Not to mention, additional core count over Ryzen, although with greater inter-die latency. Not sure what that will look like with TR3.
Is 3900X worth $500 at list over $400 3800X at list? Actually, yeah, it looks at least 25% better to me (esp. the doubled L3) if you can use the cores. The 3800X is overpriced; they probably are learning from the 1700<->1800 dynamic in gen1. Is it worth it over the 3700X at $330? Maybe not.
For me the question is really, how long will Ryzen 3000 be on the market before those better IPC/clocks/core densities show up in TR3? PCIe 4.0 support is huge; AMD wasn't anemic on PCIe channels on Zen and Zen+, and PCIe 4.0 doubles bandwidth from 3.0. Hopefully those IPC gains do not come attached to Spectre/Meltdown-like vulnerabilities. I'm excited for Zen 3 TR! That might be worth an upgrade from the 1950X. Meanwhile, it doesn't seem like Intel will get to PCIe 4 until 2020 (although that's reasonably soon).