I’ve been using a Hoodman USA “HoodLoupe Professional” for a few weeks now. It’s very helpful when using the “Live View” feature found on the latest Nikon and Canon DSLRs, providing a slightly magnified view free of any extraneous light. It delivers a clear and sharp image while eliminating all ambient light, which makes the LCD usable under difficult lighting conditions, and is especially helpful when trying to achieve critical focus.
While it doesn’t show an entire 3" LCD screen, I haven’t found this to be on issue with the Nikon D3 or Canon EOS 1Ds Mark III (both of which have 3" LCD screens)—nor is it clear that a larger and bulkier unit that would cover 3" screens would be desirable.
Up to 3 diopters of optical correction can be dialed in. It can be carried around the neck on a lanyard for convenience. I recommend it for any kind of tripod work with a Live View camera, but it could also be useful for evaluating images on cameras that lack Live View, for the same reasons (difficulty seeing the LCD under bright lighting).