The usefulness of this is just OUT OF THIS WORLD!
I've been using ParaInvite and the Pro addon for a while now, and I have to say I can't disagree more to peter_pan's review. I have tried another invites sender and I really regretted not starting with ParaInvite.
In my opinion, what really set ParaInvite apart is the ability for users to manage and resend invites. The other invite components don't have this. You find users sending out invites and forgetting about it.
ParaInvite also automatically add the invited user as a friend of the inviter. I also couldn't find this feature on any other invite components.
What *REALLY* (and I simply cannot stress this enough) increased my site's membership a thousandfold, is the badges feature. I run a fashion blogshop, and with the badges feature, my customers can place their own badges on their own blogs, inviting their own readers to come to my site. I can even have a mini-contest to see who invited more users using badges. With badges, there is no email or facebook spam, which makes it MUCH more effective!
The invites management and badges features have literally helped my site jumpstarted in terms of membership. If you look at my google analytics account, it's like the moment I installed ParaInvite, a few days later, and BOOM!
I also have SIMBunch's SIMGallery installed on another site. That is one gallery that is designed to blow off anyone's mind!!! Maybe I'll write a simgallery review when I have the time :)