The biggest issue I've had with I2P so far has been lack of content.
postman.i2p only permits torrents which includes its tracker in the torrent file, which means popular torrents from 1337x, TPB et al can't be uploaded there (at least not without changing the infohash). Torrent clients like qBittorrent and BiglyBT can cross-seed on I2P and clearnet networks which is a recent development since libtorrent 2.0 came out (software packages take a while to bump to.the latest library), but from what I've tested nearly all of the infohashes I put into my client from "clearnet" torrent sites have stalled, probably because I2P is a little too bespoke at the moment.
The potential is definitely there IMO, but unless you're just watching mainstream movies and TV it's not a replacement for clearnet/VPN.