Hello Neil -- you might be interested in my experience. I am an author who is posting his books online, unprotected, as shareware. I started, early in 2008, by asking for £1.00 ($1.65) per title from readers who enjoyed the work, but PayPal ate too much of that to make single payments worthwhile. Last summer I took the advice of another online writer and experimented with the model, asking simply for support rather than a per-title fee, but payments dwindled markedly. Recently I switched back to the original model. I think the reader prefers that: we both know exactly where we are. I increased the requested price to £1.50, and have been surprised and pleased by the response. Readers, by and large, are pretty nice people!
What I'm saying is that your offer is unlikely to be widely abused, though you might be setting the bar a bit high with "absolutely love". Why not just offer a refund if the reader is disappointed?
I don't know what royalty arrangements you are making with your authors, but one drawback of such an offer could be the extra administrative burden. You'll need a sophisticated system to keep track of it all!