I think when it comes to paid relays, what you’ll want to check prior :

Say you want to pay for relay X

1. What do non relay X users read ?

2. Are you able to read what non relay X users comment/tag on your posts? Will if make a diff if you have free relays too?

3. How’s the spam filtering done ?

4. Is there a global restriction on how many degree of users you can see to prevent spams, and are you ok with this limitation?

I think it’s also important for relays owners to have these information available for their users.

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Personally it worries me that some relays restrict viewability, as it impacts the user adoption and the integrity of the conversations held on Nostr but I think this will get better with time (I hope!)