Well I broke down and got the set because I did the math and it would have cost me more simpoints to wait and pick up the 3 premium items and 1 hairstyle I was interested in than to just buy the whole set on sale.
I was most excited for the voodoo doll, but honestly I find it the most boring of the 3 premium items. Your sim has to bind it to someone who is on the lot with them and then they can do things like bless or curse the bound sim, make them fall in love, give them a hot foot, or make them wet their pants, for example. As soon as the sim leaves the lot, the voodoo doll becomes unbound. It is kind of cumbersome to use since potentially you could be binding the voodoo doll over and over again if you want to afflict a sim outside of your active household.
The big hit for me is the cauldron. My sims will autonomously go over and conjure and I've gotten some really nice collectables out of it - deathfish, life fruit, rainbow gem, some special seeds, and 2 rainbow butterflies valued at over $1000 each! I've also gotten a bunch of lower quality collectables, but it's like having a present that you never know what you are going to find when you open it. Also, items seem to appear in the cauldron on their own. Sometimes my sims conjure and nothing shows up and other times I open up the cauldron when no one has been near it and there are random collectables in it.
The crystal ball is a close second. It seems to be pretty random whether you get a good or bad fortune. Bad fortunes seem to give an unlucky moodlet and sometimes other things happen. My sims have lost a little bit of relationship with other sims on a bad fortune day. They have also lost about $500 a couple of times. It's not a big deal for them because they have a lot of simoleons saved up, but it could make things tight if you have a family without much in the way of funds. Good fortunes give a lucky moodlet and again, other things might happen. My sims have gotten new friends and quite a few nice gifts (they show up in your family inventory). On the whole I would say more good than bad has happened for my family from using the crystal ball.
If you have a limited number of simpoints to spend, I'd personally recommend getting the baby swing or the super sleeper before spending simpoints on this set. But if you have the simpoints and like magic, this isn't a bad investment.