Well... if you want eye-candy, I'm sure there are other options. For Invoicing, I personally wanted an app that allowed me to DO something rather important (keep track of my invoices, who's paid, who hasn't, etc.) rather than LOOK AT something nice. The bottom line for me was functionality, not appearance.
Over the last few years I've tried many demos, and purchased at least two Invoicing apps (very nice-looking ones, by the way) that I've eventually ditched. Since finding PAID I haven't looked back. It's extremely well thought-out, and has allowed me to stay incredibly well organized. I even like the interface! Why? Because the app works so well.