Yeah, it is pretty cool; being free doesn’t hurt either. ??
Not sure what you mean by incorporating it into WP. Something I’m thinking of doing is to “create a page” for the subscription part, which would be in my blog template, and let the phplist pages do the rest (not in my blog template).
One of the drawbacks to phplist (aside from it’s lack of documentation) is that, at least for me, it’s rather difficult to integrate it into your own template without having to jump through hoops. I’ve decided to just put the subscribe and unsubscribe pages into my template and let everything inbetween be phplist’s template.