I like to see opening/support bands, local or not, because they are often someone I have never heard of, and this gives me a chance to get exposed to something a bit different, and maybe I'll like it.
I also like to support openers because they need the feedback from crowds too. I really get annoyed when I look around during the opener and see less than a third of the crowd there, and they are mostly just buying beers or whatever. Some openers suck, but ya gotta give them a chance...
We've opened for Máiréad Nesbitt (formerly of Celtic Woman) twice. That was a nice crowd who paid attention to us, about 500 people in both cases.
The worst experience I've had for experiencing openers was Girlschool, who opened for Deep Purple in 85 (IIRC) - they were loud, all treble/feedback, and not very interesting.
The best opener I ever saw was The Tragically Hip when they opened for David Wilcox during Frosh Week of my freshman year at Queen's University - they were a local relatively-unknown bar band, and about a year before their first EP, and a few years from exploding onto the Canadian music scene. They did a lot of Doors covers and similar, but were really great, IMHO. Was a fan from day 1.
And Wilcox was lousy, and apparently drunk. I didn't stick around much to see him.