I completed the 30day shred recently, with great results. I was starting from a base of jogging 3 times a week for 4 months. I had lost a bit of weight (I need to lose about 20kg) with the running.
I did level 1 for 2 days, then went on to level 2. It was fantastic - I don't weigh my self but I lost an entire dress size in the 30 days (I only had 2 days off) and felt amazing I am now continuing with level 2 four times a week and running 3 days a week.
I am going to keep going with this, then work up to other JM DVDs. The 20 minutes is really acheivable each day and in level 2 I am doing things I never thought I would be able to do. Me - walking push ups? Yes, I can do two sets now, starting from not being able to do more than 1 push up at a time 6 weeks ago.