I was a member. I thought the interface and site organization was great. Unfortunately, I didn't feel like it was worth $20 a month. The cost was high enough that it was noticeable when I'd go a period of time without logging in. If the cost was $10 a month, I'd most likely still be a member.
In any case, if you're interested, it's probably worth the $20 to give it a shot. If you have a lot of time to spend with it, it very well could be worth the cost.
I should note that you can get a pretty good discount if you pay a year in advance. I typically don't like those types of arrangements but if you do, you can save some cash.