Drawbacks of using a loupe:
The XIT Professional LCD Viewfinder Loupe solves the complexity of positioning, attaching and releasing the loupe by using a thin magnetic mounting plate glued to your camera that allows you to quickly and conveniently attach and release the loupe.
That said, neither Bryan nor I like the idea of gluing something to our cameras – it makes us both cringe just a bit. Bryan doesn't even like to use screen protectors. However, I like having something on my screen to protect it from scratches. That's why I already had a GGS 3rd Generation Optical Glass Screen Protector on my 5D Mark II. The GGS screen protector isn't glued to your screen. Instead, it fits over the eyepiece and extends downward to cover both the eyepiece and the LCD screen. The fit and finish are so good that it looks like a part of the camera; I rarely notice that it's there.
So why is the screen protector important? It allows you to attach the XIT Viewfinder Loup's mounting plate to the screen protector instead of the camera, thus keeping your camera glue-free.
The only downside I have experienced with using the screen protector + mounting plate combo is that if you inadvertently pull on the loupe while it's attached, stray light can get under the screen protector when viewing the LCD. Applying a small amount of pressure (pushing the loupe toward the camera) easily solves this problem.
I love having a loupe when shooting under the bright sun. The inexpensive XIT Professional 3" LCD Viewfinder Loupe has earned a permanent place in my camera bag. I don't have any experience with other viewfinder loupes, but there are others you may want to consider (like the Hoodman Hoodloupe). Keep in mind you'll need to find a viewfinder loupe that is the appropriate size for your camera's LCD screen.