I don't think a subscription is worth it unless you deal in huge volumes of fairly obscure antiques/art. I have used them a couple of times, since 99% of what Worthpoint gives you that Google can't is old auction values that don't reflect current market conditions, I don't think they add any value that common sense can't. Showing strong WorthPoint auction results might sway a few buyers who are on the fence in face to face situations. I see a lot of booths at the antique market with WP results taped onto their tough to sell items.
My general rule is: if I'm unsure on an item and it's so obscure it can only be looked up on WP, I will pass.