I went to a Brantley Gilbert concert to see Justin Moore opening. Moore was great, Gilbert sucked live.
I'm going to see Nathaniel Rateliff & The Night Sweats with opening act Margo Price. I frankly had not heard of Rateliff so I went to YouTube and was not blown away (maybe they'll change my mind in person). I'm going to see Price, who I find to be awesome.
What concerts have you attended primarily to see an opening act rather than the headliner? I went to a Willie Nelson concert to see Alison Krauss and Union Station. They were great, Willie was simply awful.