I just picked one up on e-bay: fantastic! (for rendering, no noticeable differance for the UI)
I’m using it on a laptop with an internal GTX 1060/6GB, with an old GTX 1070 in the external box.
Not exactly a laboratory test, but one of the scenes II’m working on was taking 24 minutes or so a frame on my laptop (5 odd mins a frame on my 16 core workstation with 2 x 1080tis, 8 or 9 minutes a frame on another 12 core machine with 1 x 1070ti & 1 x 1070).
The e-gpu dropped this down to about 13 minutes a frame on the laptop from 24 odd, I’d say it’s well worth it!!!
All the above rendered in 2.92 Beta - so I get (and am using) hybrid GPU+CPU with Optix, and Optix supports more shaders than 2.91.2. None of my machines have RTX cards obviously, but Optix is still way faster than CUDA usually from what I’ve found.
Obviously with most thunderbolt3 e-gpu’s sporting a 4 x pcie lane there’s a bottleneck, and I have to disable it for Davinci to run at all properly, but if you google it, redering does not usually need all the pcie bandwidth, only really using it for loading the scene and saving it.
With simple scene’s there’s not as much benefit, as in if it only takes a short time to render then a greater percentage of the time is loading to VRAM, then reading from VRAM, so the pcie x4 is more of a factor, but for anything taking a couple of minutes or so; it’s a huge benefit.
Added bonus: if you render Eevee from the command line, you get twice the throughput: one process per GPU, like I can on my full size multi gpu boxes…