Fundamentally, the plugin is sound. I didn't notice any errors or display issues.
However, restricting donation amounts to pre-defined values and no support for on-site payment processing makes this plugin less than adequate.
I have a client who's entire branding and marketing scheme is based on the $27 increments a person donates. So, being forced to use $5, $10, $20, $50, $100, $200, $500 & $1000 doesn't do him any good. I need to be able to changes those pre-defined values to $27, $54, $81, $108, $135 & so on.
Also, as much as PayPal wants people to believe their system/website is safe, conversion is king. When you take the end users, away from the original website, to make a payment on another website, conversion goes down - period!
When this plugin grows up, with abilities for the developer to designate the predefined donation amounts and to use a payment processor, behind a SSL (even if it is still PayPal), let me know. I'll be happy to give it another try!