Just as the review below me said, the app does exactly what it says it does in a very user-friendly way. Using Snow Leopard 10.6.8 and iPhoto 8.1.2 (09), I have not had any problems using iSplash from iPhoto (simply drag photo from your library to the iSplash icon on your taskbar), however some functionalities could be improved.
Opening photos in this way has resulted in the vignette in my iPhoto library not being updated despite the edits showing correctly when opened full-screen. Not a app-breaking issue, but annoying. A way Ive found of going around this is to export photo, do edits in iSplash then re-import to library, but this gets rid of the date/time stamp on the picture (which I had anticipated and prepared for)... but again, nothing really app-breaking, especially considering the quite reasonable price.
Other possible areas for improvement:
-can only navigate photos with scrolling bars (annoying when zoomed in) - a hand icon would be practical;
-can only zoom in from toolbar - again, a zoom icon could be useful if implemented;
-there seems to be a glitch when closing pictures you were editing opening up again when you switch back to the app or when using the Open function - this doesnt happen when quitting and opening the app again though.
Still, quite a good app for the reasonable price. Recommended. Now I only have to resist the urge to tweak every other picture with it!