Wow this is a pretty intense set of workouts. I have done the first 2, and I will update this review in a couple of months (like another reviewer suggested I may do another couple of weeks in phase one to make sure I'm ready).But I wanted to post my initial impressions for people who were on the fence about buying and wondering "will I be able to do it" (because I know I was :)).So a bit about my exercise background/videos. I'm a long-time video workout junkie going way back to the original Firm videos (when they were on VCR tapes lol). I will occasionally go to the gym to a class with a friend but prefer to workout to videos at home. In the recent past I have done several Beach Body sets (Chalean Extreme and Brazilian Butt), Fe Fit, some Piyo and JNL Fusion.After taking a complete workout hiatus (new house, fiance's heart surgery, moving, prepping former residence to rent out, and doing 2 jobs at work got the best of me) I started back in February slowly on my elliptical and slowly cranked up to 45 minutes. then started in on a JNL fusion rotation.I've currently done the JNL Fusion 4 times through, each of the videos. After the first leg day I was walking but with extreme pain (people at work asking "what the heck is wrong with you" lol) but after 3 days recovery, very little soreness.If you're not familiar with JNL fusion it's 30 second intervals of relatively high intensity cardio/plyo w/ 30 second intervals of strength, back and forth, for about 30 minutes. There is little core work, and few compound moves, but it's pretty fun and fast and gets you sweating. By the end of the 4th rotation I was using very few modifiers and pretty solidly doing squats and lunges w/ 10 lb weights, and 10 lb shoulder presses and bicep curls (with just a little soreness after). I can do jumping jacks for 30 seconds and burpee slowly for about 30 seconds.So wanting to kick things up a notch I ordered this and I would say that it's the perfect step up into the next level because of the addition of compound moves, core work, and the format Jillian uses of doing 3 strength moves 2 times through, then 2 types of cardio twice, then 2 thirty second intervals of different core work seem to get me gasping for breath pretty quick and I'm sweating more than I was from the JNL fusion. After I finished the second video today my muscles were shaking, but I didn't get too sore after the first day (just a little twinge to know I'm hitting a different area which I actually like).So my recommendation would be that if you can do jumping jacks, squats and lunges, and a burpee or two, you're probably ok even if slightly overweight (as I am) and it's worth a try.Also, my feeling is that if you have bad knees you will probably always want to modify to some degree but if you push it (gently) with some plyo, not to the point of pain, it can actually help. If it hurts, modify (and there is a modifier in the videos). If I start to feel knee pain I will post but I was very worried about that with JNL fusion but in fact my knees got much better when I did it.I have seen Jillian workouts that I did not care for her personality in but this one so far she's great. It's apparent she's got a good rapport with the background exercisers and is pretty funny. They're all working pretty hard and it's enjoyable to watch. I love JNL fusion but after 4 rotations through all the videos I was tiring a bit of her catch phrases and the very bad hamming it up the background people do (although I do love that she has all shapes and sizes).Overall so far, I wouldn't say I'm enjoying it but I am saying that it's day 2 and the dread factor isn't too bad. Will update.