Hey Bob,
I think that for what it does, it's probably fine. The issues I am aware of are:
* It is not, first and foremost, designed so that users can stay on your site. While going to paypal so that a user doesn't need to trust the site with their payment information is a really good option, it doesn't scream professional if it's the only option. And I think I have heard of ways that you can integrate paypal this way, but it doesn't seem like it's a highly supported option.
* The number of times that Paypal has royally screwed users for no reason and with absolutely zero recourse is, while I am sure a small percentage of the overall, nonzero. It makes me very very very nervous.
* Paying out to users via their paypal account is convenient, but it's not ACH (well, often it is, except twice). Users will need to remember to transfer the money out of their paypal account if they want to access it from their normal bank. This will increase both the effort and time required by the user.
Now, here's what I know: PayPal is the only option worth discussing for international payments at a smaller scale.
I am curious, since I expect that you have a bit knowledge on the subject, does PayPal support the 3-party payments that FPS does? This is what makes Kickstarter possible, since they as a company never need to accept your money on behalf of someone else and thus aren't opened up to a regulatory nightmare.