I am creating my first ever major plugin for Wordpress and I intend to share it to the community. It integrates paypal's subscription button which is very useful for people who wishe to build a membership site using Wordpress. Question: Is IPN reliable enough that there's no need to control the subscription thru cron myself? I have not tested my plugin for a span of a month or so though because doing so would eat a lot of my time. 
Is paypal's IPN reliable enough?
paypal
Yes, it is reliable enough. Thousands of businesses depend on it and it just doesn't fail. Now, is the plug in reliable? That's something you need to research more.