I signed up, downloaded a few stock arts, forgot about the trial, used the stock for my cover, loved it, got charged for the sub, complained and asked for a refund, got a refund.
So yeah, I'd say it's worth it.
If you know what kinds of stock you need, imo, go find all of it first before you sign up. Bookmark it all.
Then sign up, download 5 per day (what the trial gives) and get your full 35 stock pieces out of the trial. Cancel on time. Don't be lame like me and forget.
Then you have 35 pieces of quality stock to use for your covers/website. Why not?