I'm well aware that this is an older review, originally posted 2010-05-07, but I'm migrating all of my reviews to Deskthority.
Pictures coming later.
So, this keyboard is ridiculously heavy. FedEx says, with a thin layer of bubble wrap and a barely big enough cardboard box, that it weighs 13 pounds. I fully believe that figure.
Keycaps are nicely shaped and textured.
Key travel is actually less than I was expecting, and I think it feels slightly shorter than a Model M. The British patent said about .15" key travel, which is 3.8 mm - so it should be relatively short, though.
The keys are quite light, pretty smooth, and nicely tactile and clicky. The linear spring does add some resistance to it that I could do without, but it feels good. I do find it MUCH easier to avoid bottoming out.
Sound is more clear than a Model M. As has been said, they don't ping like a buckling spring board, they hum - you can even feel the hum in the keys.
Spacebar is RIDICULOUSLY STIFF. That's something I'm going to want to mod. Edit: Spacebar is actually not too stiff at all, once the stabilizer bar was removed and then reseated.
This will be updated with impressions of how it actually works.
There was another 5251-keyboard, a 66-key Data Entry model. Aside from the stuff in the announcement letters, I can't find any mention of it. I suspect it may just have been the 3277 keyboard with a different layout.