PiKVM seems to be the large competitor here and is completely open source. If you're looking into KVM solutions, probably check it out, but JetKVM is over 50% less, which is a huge argument in favor of it.
What justifies the V4 Plus being worth $350? They're using the CM4 so they’ve made a PCB, but what hardware are they adding over the peripherals available on a Pi 4/5? All I can tell is an additional Ethernet port, a SIM card tray, and an “ATX controller”.
What does the board look like, why can’t I DIY that version, etc. Are they just trying to make it up with the software (that I also can’t tell what it looks like).
It's not really worth that much. You absolutely could DIY it, probably just kludge in a basic $30 HDMI capture card. Also JetKVM is now just as "open-source" as PiKVM is, so there's not even a moral high ground to spending extra. Both are open-source software but not open-source firmware or hardware (no schematics or gerbers or anything like that available).
The JetKVM is very impressive looking at a great price. Until recently it wasn't really available in the US but it looks like it is now/
The V4 Mini is a very nice piece of hardware. I paid $300 for one in April from Amazon. I also got PiKVM running on a Pi Zero 2 W and it worked fine but was a bit squirrely. Having the purpose-built device is nice.
You can also use a Pi Zero 2 W as a serial console: it has a USB On-the-Go port perfect for the purpose. But the KVM approach is more generally useful since you can access a consumer BIOS from it.
They recently opened a global store. Previously, the only way to get one was to "buy" it on kickstarter, presumably from the US as well as the rest of the world.
over 50% less the price, I see the JetKVM at $90 USD, but PiKVMs range from $230+.
I found PiKVM useful as I already had the hardware laying around, so setting one up didn't cost me anything, and its a pretty good experience. If I were to buy new though, not sure I'd find it worth the cost for my use case.
https://pikvm.org