I really like the 3d effects for the 3ds. Its subtle. But it certainly gives me a headache...

The level of the 3D effect really depends on the game. Pilotwings and Street Fighter really seem to stand out big time, same with Steel Diver and Nintendogs (at the right moments). Also depends on your individual vision. Some people with great eyesight still might not see it as well as someone with medium to bad vision. I'm not entirely sure how it works, but I can see the depth perception quite well.

3D in general doesn't add to gameplay, but for some games, it does make certain tasks a little bit easier to not screw up (such as flying through rings, hitting a block within a 3-Dimensional world, and judging the proper distance between two cars), so in a way, it adds to the enjoyment of already established gameplay.

Take Metroid: Other M for example. In that game, sometimes its really hard to judge what's an actual platform, as opposed to just the background itself. 3D in a game like that could really help when you're jumping around. So again, it doesn't technically add to gameplay, but it definitely makes certain annoying gameplay elements  drastically more enjoyable and easier.

There seem to be plenty of instances where the 3DS has pop out effects. The pencils in the Activity Log screen pop out. The dragon boss in AR Games pops out when he attempts to bite you. There will be more. I'm more impressed with the depth that's possible. I have Street Fighter and Ghost Recon. Street Fighter feels a bit tacked on. It's not remarkably impressive. Ghost Recon on the other hand looks like an amazing diorama of depth. I can count at least 20 different levels of depth in the canyon portions of the game. I can see it being very impressive in any top down games like RPGs, Zeldas and Turn Based Strategy. In the case of Ghost Recon specifically, it actually does help with the gameplay, because different levels of depth affect shooting ability, line of sight, etc. and whenever I turn the 3D off, it's completely muddled and difficult to judge.