Hattie's Hat was a great experience. I never hang out in Ballard, but we were going to a show down at the Sunset and needed a place to crash beforehand. We met up with a couple friends and sat in one of the half circle booths, which was probably the best idea of night next to ordering a round of Pabst Tall Boys.
I really enjoyed Hattie's because of the ambiance. I didn't get the feeling that it was a trendy, scenester bar. It just sort of was and you either liked it or didn't. The interior at night is dark, very dark. There is random stuff all over the walls -- artifacts that look like they've been there since the beginning of the bar.
The waiter was really cool, but really slammed. The place was busy. I saw people in there from all walks of life, which I appreciated. It's definitely not a classy upscale wine bar so don't expect that. Expect a great place with a great vibe. My advice would be to go with some good friends and shoot the breeze all night.
One tip: Get the Pabst Tall Boys. Even if you aren't a cheap beer drink just do it for the experience.
Another tip: Hattie's is a great place to hang out before going to see a show at The Sunset or any other place on that strip. Spend about an hour there beforehand and loosen up with a drink or two.