I like that it's very simple, quick & painless - no complicated integrations or 3rd party software required! It also makes managing invoices so much easier than manually entering them in Excel/Google Sheets etc.
Also being able to send out an email with all my customers' details attached at once was quite handy too as some of our clients are overseas (and not always super tech savvy). Nothing really comes across negatively about this product but perhaps more options could be added such as customising payment reminders? We've been using bookpi since we started up over 2 years ago now which has proven extremely useful when sending bulk payments via paypal / direct bank transfer without having to enter each customer individually into excel first.