Experience isn't very different yet. In theory spam reduction since there is cost to join. Will be interesting if clients add relay-specific feeds.

Ideally, when clients start to get smart about relay querying, paid relays will be a great source of spam-free content to be queried for when you don't know what exactly you're looking for. Some badly-written thoughts on this here: https://fiatjaf.com/3f106d31.html