I've had the Nuvi 500/550 RAM mount for a couple of days now, and am quite happy with it.
The package includes a Nuvi 500/550 cradle (similar, though not identical, to the one in the image accompanying the description; the one I received has a moving hinge-like mechanism to lock the Nuvi in place), a male connector that must be attached to the cradle with included screws/nuts, the female connector for the handlebars, and a few other pieces of mounting hardware (a piece of neoprene adhesive foam to improve grip on the handlebars, and so on).
I found a video on Youtube that gives an accurate demonstration of how it all fits together: [...] You'll see that the Nuvi and its cradle pop off easily, leaving only a small, anonymous connector on the bicycle handlebars. With the cradle I received, you could alternatively eject the Nuvi and leave the whole cradle attached to the handlebars, too.
One advantage I think this product has over the ball/socket handlebar mounts for sale on Amazon and elsewhere is that the Nuvi is very securely held in one position. With ball/socket mounts, I suspect a few bumps or curbs would cause the relatively heavy Nuvi to sag or shift position.
In operation, the product seems to be great. Assembly was slightly complicated by the fact that the nuts they provide are too small to lock in place inside the hexagonal holes. As a result, you'll need both a cross-head screwdriver AND a socket wrench to tighten it properly. I solved the problem by installing the screws with the nuts on the inside, rather than the outside.
So far, I'm very pleased. I expect no trouble with this product, and am looking forward to having my Nuvi as my "dashboard" as I go for some longer bike rides in the near future.