I would like to know if the 100% viewfinder number mentioned in many cases to praise the Canon 7D and other similarly-priced SLRs is a really meaningful figure.
How does it actually affect the photographer's user experience?
In my experience, transitioning from 95% to 100% made a significant difference in my photography.
The 5% can hide a decent amount. Shooting a lot of wide-angle means that there can be a lot in the missing 5%. It's easy with my 10-22mm on my 7D to capture the foot of my tripod in that extra 5% and I'd rather not crop the image.
You can't easily judge what you're missing. Remember, lenses don't see the same way we do. Unlike the human eye, that 5% in the lens will change in size with every focal length change. Just in the studio setting, 85mm will maybe grab an inch of more background, while 30mm could grab a foot (not measured, but an example). You could be off your background and you wouldn't even know it until you see the image.
Information is power. Knowing what is in the extra 5% can only benefit you. Does it impact the user experience? Definitely. There are many things to consider when taking a photograph. Not having to worry about the extra 5% lets you focus on the other things. In nature photography, I had to always think about whether I would get that road in the extra 5%, potentially causing me to crop. In architecture, I need to know if I can keep empty space around the building or if the neighboring building will block it. With 100%, I can see that and simply move, whereas 95%, I'm chimping.
Make it right in the camera. For those saying that you can simply crop to the viewfinder, don't listen to them. Every tool that helps you capture the picture that you intended to capture is a benefit. A benefit that completely eliminates a potential need for post production is a big win.
Is it worth the money? That's the million dollar question. It's up to you.
An extra benefit that you may find more important than the extra 5% of viewfinder with the 7D is the brightness and size of the viewfinder itself. Composing in image in the 7D viewfinder is heaven compared to the 50D. This only pertains to the 7D, though. I don't know if this is common in 100% viewfinders.
My opinion? There are many other factors in a camera to consider besides 100% viewfinder. I will say this though, when you want to test out a camera, usually the first thing you do is pick it up and look through the viewfinder. I was sold based on the viewfinder alone.
The percentage dictates how much of the capture scene is visible to you before the capture. A true WYSIWYG situation is a 100% viewfinder because what you see with your eye will be what your final image gets. Anything less than 100% means that there are elements that will be captured in the scene that you cannot see at the time you look through the viewfinder.
The importance of that depends on the photographer. For the average person, that's probably not a huge issue because they often center frame and aren't as worried about the composition as they are the moment. For professionals, or those more focussed on image structure, a 100% viewfinder means that they don't have surprises and can be assured that the scene they view is the scene they get. So, judge that requirement based on your need. I, for one, much prefer having a 100% view...
There is only one number which is important: 100%
Anything less prevents from properly framing your shots. If what goes in your frame is important to you then you have to get a camera with a 100% coverage viewfinder.
If you shoot with a cropped viewfinder, typically around 95%, you will be getting 5% unexpected things in your frame. You can guess and zoom in for to attempt and compensate but it is extremely difficult to do so precisely. As Jay Maisel says: Everything in your frame either helps or hurts you.
A 100% coverage viewfinder has long been a high-end feature and a camera with one used to cost several thousands but as more and more people become more interested in photography, it slowly trickled down in price into the sub-$1000 range.
Once you get used to a 100% coverage viewfinder, it is very frustrating to use a camera with a cropped viewfinder. Digitally it is much easier to show a 100% coverage image and all SLDs show 100% view. Most DSLRs now also have Live-View which, with the exception of Sony DSLRs and one defunct Olympus model (depending on the mode), all show 100% in Live-View, yet people have good reasons not to use Live-View all the time, so it is not a replacement for a 100% coverage viewfinder.