Bugger Cardiff for Hay-on-Wye?

I’m on the phone with Joe E. We’ve been gabbing about travel gear, including the DreamSack, and then we got to talking my draft itinerary. Joe says Cardiff itself isn't worth 3 days. (Well, he didn't quite put it that way, but I'm trying to keep this post PG-13. Somewhat.)

Hay-on-Wye, Joe says, is much more worth some time. It's near Hereford, and has “pretty much nothing but used bookstores”.
Any bookstore there, will give you a map to all the bookstores. (I’ll miss their May 23-June 1 literary festival though.) Then you raid pastry shops for somthing to eat. There’s also a good campground. Sounds like something I’m definitely interested in, so maybe I can piece that in as a day/side trip?
Joe also recommends joining Hostels of Europe. You get discount on many places, and it’s not as popular a membership as HI. But the Hostels of Europe properties tend to be much cooler places, Joe says.