Having looked at various resistance bands I opted for these, the medium to extra heavy ones. Sadly when they arrived I was disappointed, as even the extra heavy one is very easily stretched, and then stays scrunched up and doesn't return to it's original flat shape unless you actually smooth it out. The lighter ones aren't of much use (for us), unless you fold them in half and double them up, and we're by no means strong.
We bought these after trying some that our personal trainer bought. Unfortunately he couldn't remember where he got his from, but sadly these don't come anywhere near like the quality of his, even though he indicated that prices were similar.