First, what are flex bands? Flex bands are giant rubber bands that come in various resistances ranging from 15 pounds to 200 pounds. These aren't your bands with the handles on them that you find in any retail store, flex bands are a whole new exercise experience! The uses for these bands are endless and people on any fitness level can use them, from beginner to advanced. I used these when training for college football and I use them for powerlifting. With the same bands that i use in advanced weight training my mom and my fiance use for their everyday fitness needs.
If you are just looking for a workout at home then I recommend getting the 15 to 25 pound resistance band which is $12.00 from Jumpstretch.com. Considering people spend hundreds of dollars on gimmicks and gym memberships, $12 is a heck of a deal. With one band you can work your shoulders, biceps, chest, triceps, abs, lower back, and all the muscles in your legs.
For the intermiediate to advanced weight lifter the average and heavy bands will take you to a whole new level of muscle development and strength. Try wrapping a pair of average bands to each side of a bar when squating or benching.
The uses are endless for these amazing exercise tools!
Update On Jul 18, 2009: I really believe I am as strong as I am today because of these bands. Before I used them I was just an average lifter, maybe a little above average. Now my strength has increased so much that I'm breaking powerlifting squat, bench, and deadlift records. I squat over 600, bench press mid 400's, and deadlift 635 all unassisted by any gear and no hormones. I can honestly say that I would not be at these numbers if it weren't for these bands. Try them yourself and you'll see what difference they can make.