They do take a massive depreciation hit, so yeah, big discounts on grey market/nearly new.
I really like the style, and the finishing is excellent: hand-engraved balance cocks (no two alike) etc etc. I like them. They have a strong house style with a lot of jazzy art deco references rather than the much-referenced Bauhaus (see above!), and are definitely worth a look in their own right, IMHO.
As CWI just said, the "Pano" models do look quite a lot like a Lange 1 in concept, and that's a common complaint. To be fair, though, GO actually have a cooler way of doing the "big date" with a completely different mechanism that makes the two numbers align better. Also, in one way or another, they were all the same company twenty years ago. So I think it's little unfair to call GO a derivative or imitator of Lange: of course some of the design elements are the same, as they're often referencing the same back catalogue. But it's really only the Pano/Lange1 similarity that projects that impression, I think.
So in conclusion, I'd say buy it if you like it and if the price is right. But if you want something that doesn't look at all like a Lange, avoid the Pano. Try the Senator range - the classic Senator three hand is lovely I think, and the perpetual calendar too. Also the "Sixties" and "Seventies" lines - more divisive, you'll love or hate either, but very individual.
P.S. Loving the conversation turning to the German catalogue, a weird obsession of mine. I like a lot of the GO's, Pano included, even though Lange is probably my grail (about five of them at least!). For now, Sinn, Stowa etc. are a more accessible possibility. Watch this space. ;)