It would be a good idea to recommend to users that they set up a separate Paypal account for paid subscriptions. Doesn't cost anything, and would avoid several problems.
If user enables Paypal IPN, they'll receive IPNs for payments into the account that are unrelated to paid subs. Doesn't cause a glitch, but the subs handler throws an error for 'unknown transaction type'. It's not necessary to have IPN enabled - Subscriptions-Paypal.php sends a notify_url, which overrides IPN settings and forces an IPN post to subscriptions.php. Unfortunately, admin settings directs users to enable IPN.
Recurring payments are not well understood, and caused the problem discussed here, and a different one on another forum I'm working on. The recurring payments are not initiated by SMF, they're handled by Paypal without any input from the forum, and use the data that's in Paypal's database from the initial subscription. This can cause problems if the forum url changes, the Paypal account is changed in subscription settings, or if the forum cleans up the member database or otherwise deletes members. Paypal will continue charging renewals for members that are long gone. I don't know if there's a way to cancel recurring subs in SMF, but we've been doing it in the user's Paypal account.
Paid subs generates a lot of errors for a lot of reasons, and it's difficult to determine what's happening unless you've spent a lot of time tinkering with it. The front end looks simple and works well, but it's actually doing some complicated things going back and forth with Paypal, and there really should be some decent documentation for forum owners.