In their time they were very good and marked a welcome shift to many.
You have to understand the state of American music when Guns & Roses came onto the scene. With the rise of MTV, popular music became more artist and image-driven than sound-driven. Your listening choices seemed limited to heavily-produced music dominated by synthesizers, "college music" featuring a more "artsy" style or older acts making comebacks, but with more folksy or experimental works (think Paul Simon's "Graceland").
When the opening riff of "Sweet Child of Mine" first played on the radio people actually froze in their tracks to listen. They didn't dance...they didn't tap their feet...they just stood and listened. Then, as the rest of the song played, listeners in their teens and early 20s thought to themselves "this seems so familiar and yet so new."
A lead guitar, rhythm guitar, bass and drums...fronted by a powerful voice. What did they call this thing? Oh yeah, it's called ROCK & ROLL...but it's not the dated stuff on the oldies station, or the heavy stuff churned out by the aging metal crowd. This was good ol' rock but with a fresh edge.
In a time when "pop rap" was gaining steam with acts like the Beastie Boys and Run DMC, Guns N' Roses helped reset American rock roots and showed that, once again, it could be both harder-edged and mainstream.