I did it before Christmas - 5 days a week for 6 weeks. I had been doing some exercise before I started - some weights and some cardio. I found that each level started very hard and got easier as I got to day 5ish. Lots of people say that level 2 is harder than 3 - I think they are similar but starting 3 is hard again as it is different.
I lost 8lb and 1 inch from my waist. I am still better at press ups than I was before I did it! It made my knee hurt but I couldn't tell you why. level 2 and 3 have lots of time in plank position doing other exercises. I found Jillian motivating but I think some people find her annoying. For a blow by blow account of my experience and to join some others who have just started look under fitness and exercise on the main forum for a thread called - "anyone up for a 30 day shred challenge."
I really believe that it should not be done daily - if you work a group of muscles you are meant to rest them for 48 hours. But 30 consecutive days has a ring to it for marketing purposes.
It did hurt but not as badly as I expected and I think it is worth it. Good luck!