Ive been torrenting on I2P with qBittorrent for a lil while now, its not as fast as the clearnet, the fastest speed down ive ever got so far is 1.7MB/s, and this was on a torrent with loads of seeders. Its possible to increase your tunnel quantity and lower the amount of hops each tunnel has for better performance, but you will have less anonymity with less tunnel length.

But so far its been nice, there is no need to think about NAT/Firewall, as all peers can communicate with each other, but it doesnt have as much content as the clearnet, so I try to cross-seed what I can. But when im torrenting, I try to go I2P-first, and then fallback on the clearnet if I couldnt find what I was looking for.