I have about seven ISO 400 reels of 36 shots each, all of which have a use-by date of sometime in 2006.
Are they useless now and should I just throw them away?
Or, does the Use By date not matter and it would not make a difference if I used these reels to take pictures?
Absolutely. In some cases, people do this for the effect that is achieved. Flickr, for example, has a group dedicated to images from expired film. Some of the outcome is really quite nice, but it is obviously going to be hit or miss since the nature of degradation is unpredictable.
Also, film expiration is not absolute. It largely depends on storage conditions because the number there is basically a guideline for film that is on a store shelf at room temperature. If you've been storing it refridgerated, or better yet frozen, then it may well be perfectly fine. Basically, unless it's really old or kept in a high temperature or humid environment, it should be fine. And, if it isn't, then see my first paragraph and enjoy it anyways. :)
That film can certainly still make images, but some things could be a little off. It was stored at rather high temperature, so expect some degradation. If it's color film (you didn't say), then the color ballance is probably the biggest change you'll notice. The next effect is loss of ISO film speed. The main reason for color ballance problems is that the sensitivity of the various layers don't degrade equally. If in doubt, take a color reference shot for each lighting condition. You're only 5 years past the use date, so I doubt you'll see a major loss (like a f-stop or more) of ISO.
A long time ago, I found a whole bulk role of some old black and white film that had been sitting forgotten in a box for 25 years. It was a lot older than I was at the time. I made a short roll and did some ISO (called ASA back then) tests, and determined it looked pretty good at 8-16. Yes, the film was very slow even by the standards of the day, but it was great for a kid to screw around with and learn some photography. I even took a few decent pictures with it. It was great for learning to judge exposure. This was back before cameras commonly had light meters (we also had to trudge to school barefoot in the snow uphill both ways, etc, etc), and you had to actually know something about exposure, judge the lighting and the subject, and hope you guessed right.
Anyway, the point is that after adjusting for the much degraded ISO, it worked fine.
The part of the film most exposed to the environment is on the outside of the spiraled roll tucked into the reel. It's therefore a reasonable test to withdraw a small amount in a changing reel, clip it off, load it into a tank, and develop and briefly fix it. If the film comes out clear, or perhaps with some periodic marks along the edges (from stray light that leaked), it should be fine. If it's cloudy or is marked in any way, you're likely to be disappointed if you use the rest of the batch.
If the film is color, consider sending a single unexposed roll out for development. (If it's color negatives and not slides, request that prints not be made.) If it turns out clear, you have wasted 1/7 of your film, thereby increasing the development cost of the remaining rolls by up to 1/6 (or much less if it's print film). If it turns out bad, though, you have saved yourself the effort and disappointment of exposing one (or up to six) rolls only to have the pictures ruined. Fairly cheap insurance, all things considered.
