"Interactive Interpolation is a better means to add resolution to your images for the purposes of enlargement. The bicubic method used by Photoshop when you select Image > Image Size... and adjust the resolution of your image often leaves artefacts that are visually undesirable in the final print and can soften the image to unacceptable levels. Interactive Interpolation allows you to adjust the final output of the image in a preview before interpolating the whole image." - from their web site.
It's that ability to preview the image, and adjust the settings accordingly, that make this a valuable addition to your Photoshop tools.
There's a helpful walk through on their web site, so I'll skip doing one here. Download the fully functioning time limited demo and check it out for yourself. Once you try it, you'll be hooked. You'll also love Refocus and the Detail Sharpener which are part of the package.
Final verdict: a well thought out product that is quite worth the purchase price.