I have the medium and the heavy. The medium does work to start on (assuming you aren't already shooting really heavy bows), but by the time I bought the 'heavy' I was using medium completely looped around for double the resistance. The heavy I currently use half-wrapped around so it pulls pretty good.
If you want to upgrade, you can order replacement bands to save yourself a few bucks... just get a replacement band that is the next step up.
Like amar911 mentioned, they're good for a number of exercises, if you are creative enough. I use the medium weight now for diagonal pulls to help strengthen a few of the other shoulder muscles and ligaments. And they're good for warming up before hunting or shooting.
I'd say go for it.