I enjoyed 30 Day Shred as a change of pace. I did not find her strength work particularly challenging. The second time I did this workout I used it more as a cardio and abs workout. I did the cardio sections twice. I enjoyed her Banish Fat, Boost Metabolism DVD more.
I love Jillian, but her workouts aren't intense enough for me. I modify everything to make them more intense. They do break up the schedule, though and are a nice change of pace sometimes.
I am curious which ones you modify, and exactly how you are modifying them. I think they are good how they are, for what they are intended. I think Jillian uses 3 lb dumbbells, and I use 5 lb dumbbells, but other than that it's the moves themselves and the combinations that work so well.
The best part of all of them, in my opinion, are all of her combinations and variations of plank moves.