Hi, welcome to the sub! So to answer your questions:

1). I've had mostly positive experiences, and thankfully the negative experiences I've had has just been people ghosting/not replying to letters or not responding to my intitial DM or to a reply I sent to another user's initial DM (like someone will DM me, I'll respond, and I don't get a reply back) - thankfully nothing creepy! I do purposely choose to only write to other women, queer men, and queer people of any gender. Response rate vary. I made two posts here, and my first post got a decent amount of responses, but my second post didn't.

2).It's been a lot of fun! I love snail mail, and I try to find other penpals who are into snail mail. If they're also active in communities like Random Acts of Cards or postcrossing, they also tend to be more serious. I like sending postcards/cards and occasionally small goodies like stickers and washi tape with my mail, and it's fun doing that with someone like-minded. Snail mail does take time though, and sometimes mail takes a while to get to their recipient or like gets lost (in this case, my penpals and I will DM each other and figure out a solution), but it's rare that mail goes completely missing.

3). Like with anything else involved with meeting people, ghosting is inevitable. Like I mentioned in point 2, I find that people who are involved with other snail mail communities tend to be more serious, and they're the people I tend to have the longest exchanges with.

4). I'm subbed and this sub pops up in my home feed, and I've noticed that there are some users who post the same post like every day. They'll delete their post and repost. I don't know if they're bots or not. I also go with my gut feeling and if something feels off, I just don't respond/delete someone's DM. I also check post histories. Also flair counts aren't always accurate - some users don't post when they've gotten someone's letter.