I used heavy duty resistance bands during my workout today and it makes the exercises much more difficult. I specifically used them for squatting and bench pressing. It didn't necessarily make the lifts heavier (although I used less weight than usual) as much as makes them more difficult.
Here is a video of what I'm talking about
Make sure you pick up a pair to add a new dimension of difficulty to your workout. If you are a personal trainer (I am) you MUST add this to your repertoire. I bought mine off of workoutz.com and you should too. Here is the link to the workoutz.com product page. Live well.