I haven't purchased from them, but those vouchers are everywhere. So that gives you an idea of what they think the wines are really worth. I've read very mixed reviews - most very similar to what @Stressbaby posted. Some good, some you spit out. On average $8-12/bottle retail. Not much of a bargain, but probably not a rip off either, if you're getting the discounts. One advantage is it'll open you up to wines you can't get at your local store.
Not really a big fan. They hype it up, giving you the belief that you are getting a great deal on high quality wines. It is simply not the case. with just a few exceptions, the wines sent to me were average at best.
I was a Member of the K&L Wine Club for almost 18 months IIRC. They have multiple levels of wines from $20 a month (for 2 bottles) to $50 a month. You can reorder any you like at a discounted club price. They send a variety of wines. No tax usually and shipping was reasonable. Only reason I cancelled was I wanted to concentrate on wines from WA/OR for myself but all wines I had were good/very good and I was in the $20 a month level.