It arrived today. It does exactly what it says on the tin of course, and seems to do it quite well! The seller very kindly included some quite high-quality-looking Coax pre-terminated as BNC to RCA converters, and I haven't noticed any degradation of the picture at all when using it.
The thing also seems to be able to pass through PAL signals unscathed. Obviously as stated above it doesn't magically generate captions from nowhere, but neither does it ruin the signal which would make it necessary to unplug the decoder when passing PAL!
However, it does NOT passively pass through the signal; the device must be powered in order for the signal to reach the TV.
It has a nice easy 12V DC barrel plug input so I can find a power brick with a proper plug and not one of these flimsy things from across the pond soon enough
The main problem I can see with it is the lack of configurability. Specifically there's no way to adjust the position of the captions, so with a widescreen TV you'll want some way of shifting the zoomed image, and with a 1.85:1 film it looks like it'll be a very tight squeeze and might involve either shaving the very bottom off the captions or the very top off the frame. I haven't quite had a chance to test this with a 1.85:1 film yet. In terms of its configurability, you can basically have captions on or off, and you can select which CC/TT (Teletext?) channel to view (I'm guessing the channels besides CC1 aren't used much in practice). There's also an option to have the visibility of captions triggered by an external signal so you could attach an easy toggle switch or even build a remote control if you were motivated enough (I'm definitely not myself!).