Something I've been thinking about lately given the 2016 season of South Park is kinda exploring this very topic. In the case of South Park one way they're doing it is with a fictional fruit called "Member Berries," which are talking berries that constantly reminisce about "nostalgic" things. For the most part the berries seem to come off as a good thing, making anyone who ingests them feel good as they remember the "good ol' days" which usually involves pop culture like the original Star Wars trilogy, or just past decades in general.
However it's also shown the berries have a "dark side" to them as sometimes the "nostalgic" things the berries might suddenly start talking about is stuff like "member when marriage was just between a man and woman?" or "member when this country wasn't so full of immigrants?" This basically shows nostalgia isn't really an inherently good thing, as for some people their idea of "the good ol' days" are things like "I miss when I could call a nigger a nigger and they couldn't use the same bathrooms as me" or "I miss when it was socially acceptable to bully people for being gay or trannies."
I've got a couple personal pros and cons of my own that differ from the way South Park is exploring the topic.
Pros:
-It can help give some temporary comfort when you're feeling down, as you're remembering things that filled you with joy
-It can help you re-discover things from years ago and you can either re-experience the same joy you felt back then, or maybe enjoy it more than used to
Cons:
-If you're too "stuck in the past" you might not be able to truly move on with your life
-Sometimes the opposite of my second "pro" might happen, where you re-explore something from years past and you suddenly find you don't like it anymore, making you question why you enjoyed it in the first place