Having been a DL fan and having seen them many times over the past 35+ years, I can say that at this point they are statues and dinosaurs that play the same songs the same way with very little energy. They don't seem to be all that interested and it's clear they are in it for the paycheck. My wife loves them and we go every time they come around and they LITERALLY have played the same exact set list every time. For people that haven't seen them before, they might be ok, but for folks that have seen them over the years, be prepared to be disappointed. To me, they seem like they are just going through the motions these days.
I also have seen Poison multiple time over the years, the last time opening for DL. The show was a Tesla - Poison - DL trifecta and DL was clearly the weakest link of the three acts. It was the first Poison gig in like 6 years and they were fantastic. Bret was a madman running all over the stage, really putting a lot of emotion and effort behind the performance, and he truly looked like he would have played for free because he just missed performing. After Poison finished up, Michaels stuck around while they set up for DL, hanging out with fans, high fiving well wishers, signing autographs, taking selfies, etc. with his feet hanging off the end of the stage. It was a total love fest. When DL started their set, security had to come pull Bret off from the stage. He definitely was way more into it than Def Leppard was.
Who knows if that's the current state of affairs for those two bands, but that's how I saw things two and a half years ago.