A7X... The headliners of the show were them and Flogging Molly. I didn't have high hopes for Flogging Molly and intended to leave early if they were last, but they were first and absolutely amazed me. Then A7X came out... they were just so unbelievably boring...
Oh, and Breaking Benjamin. They headlined a concert in my hometown and... it was just... not good, at all.
I've seen flogging molly a fair few times, and on at least 3 of those occasions, the bass player has passed me a tinnie of Guinness. Bass player equivilant of the spidey sense?

They have always kicked ass when I've seen them.
Worst band, without a doubt has to be Rancid. I used to be a huge Rancid fan, certainly one of the reasons I wanted to start playing bass 9-10 years back. Saw them in 2003, and they were just terrible.
I've heard high school bands sounding much tighter than that. All the support bands just blew them out the water.
Worst audience had to be a few months back. I went to see Sick of it All playing, and while I admit, it isn't going to be for everyone, they are one hell of a live band. They were flat out amazing, right into it as they always are. The kids (well, teenagers) who were clearly there to see AFI just sort of stood there jaw dropped. There was about 30-40 people who had clearly gone there to see SOIA and were enjoying it, but we kept getting dodgy looks from the AFI fans
Youth of today, I just don't know!
I also expected AFI to be terrible, I saw them in 2001 and loved the show and was a big fan at that point, but when the albums after the Art of Drowning came out, it just didn't appeal to me anymore. Luckily they played a huge range of tunes, old and new, live and really nailed it
