From what I remember, it's very slow and has little activity, except maybe among Russians.

The protocol itself is nice, and the community isn't half bad from what I've seen, but... yeah, if you're want to join some forums or imageboards or whatever there, you pretty much have two choices: speak Russian or suffer. I do attest to the Russian I2P imageboards being great though. :>

I heard that there are some IRC servers on I2P as well, but I haven't joined any of those, so I can't speak for how active or interesting they are.

The ability to torrent with it is neat and the fact that qbittorrent supports I2P now even more so. I feel like it finds more adoption among hobbyists and regular people who want to be anonymous rather than criminals which is a good thing (although this seems to change in recent years as more criminal and criminal-adjacent websites like dread move to it).

The biggest problem for me is that there is no real browser for I2P and all the projects offers is merely a Firefox with changed proxy settings. Either they do it like Tor and actually change the browser substantially in order to make it suitable for browsing I2P or they leave it to the users. Unfortunately leaving it to the users opens doors to plenty of OPSEC mistakes. Even Tor has a huge security problem where the browser isn't even half as secure as it would need to be due to pandering to normalfags needs who don't care about security. I see the same problem in I2Ps case where they want to use a full-blown Firefox without any security considerations.