I have been a member for a few months and I have 7 pieces of Jewelmint jewelry so far.
I love, love LOVE the designs and the little back-stories behind each piece of jewelry is really fun... But I won't lie, the quality is not great. I mean it's okay, but definitely nothing special and I think the pieces are extremely overpriced for what they are (reminds me of Forever 21/Target jewelry but just a lot more expensive). I have made all my purchases on the site with the livingsocial.com deal (they usually have one once a month and you get 2 pieces for $25) I would never ever pay $30 per piece. The jewelry looks great at first, but after a few wears the gold/silver plating seems to ware off and there's other quality issues. Out of the 7 pieces I got, I've had problems with 5. Not great. However, I will say that their customer service is very good and they do not hesitate to do returns/exchanges and they give you a pre-paid shipping label.
As far as cancelling membership I'm not sure. But they say on the website all you have to do is call or email them and they will do so. I'm also not sure about skipping months. I want to say you can skip as many months as you want, but I'm not 100% sure. You can call their CS and ask- they're extremely helpful!
Hope this helps!!