The only workout I really regretted buying was Chalene Xtreme, and only because I don't like her counts compared to Cathe. But I sent it back for a full refund, so no major regrets.
In the 90's I bought Billy Blanks dvds and never stuck with them. Through them out years ago!
I have quite a few Firm workouts collecting dust, but I still got use out them because that is really what jump started me into working out and then I graduated to Cathe. So really no regrets. I hold on to them because I always think about what I would do in 10/20/30 years. Will I still be working out? If so, I like them because I could probably do them if I'm in shape in my 60's or 70's...lol. I know weird
ETA: I forgot one of Cathe's! I actually regret getting the whole STS series. I tried to start it, but it contains too many equipment changes that I really don't have room for in my tiny condo. Also, the 1 rep max was too much thinking for me...lol and it seems I would need to buy heavier weights I also have no room for. So those sit in the boxes they came in and I use her Gym Style, Push/Pull and Pyramid series and still get great results. But I pretty much love everything else of hers.
I have to disagree with most though, I loved Chalene's Turbo Jam and Turbo Fire series. It really works you! I love her form in it and have actually incorporated it into Cathe's kickboxing workouts. Chalene really knows how to get the core area which I like to work extra hard. Unfortunately, I had to stop for awhile due to tennis elbow