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.