I use iWatermark to create web-compliant pictures so i use it mostly to reduce the size of pics and add a watermark.
So this little software does what it's supposed to do and it does it very well and in a fast way.

User Interface: The frontend is clean and easy to handle. Once the options are set for resizing, thumbnailing, watermarking, file-type and renaming you can use iWatermark just from the startscreen without the need to go to any detail screen, that makes it very handy.
Also fine that the UI looks and feels the same on an Windows PC (no worry, i just use it sometimes)

Features: The creation of different pre-defined watermark-sets is a great feature i use very often because i manage a couple of sites and so it's very easy to resize/watermark the pics in one step for the particular site. The watermark-editor works fine, you can define pics and/or text and combine it in one watermark with some effects if you like. The preview funtionality gives you an good impression what the image will look like. The "scale" option to constrain the watermark to a specific percentage of the image is a great feature, especially if you have a lot of portrait and landscape pics.

Feature request: what i would love to see in an future release is the possibility to use different positions for watermark-pic and text (for example: text bottom right, and pic in upper left corner) or the use of more than one watermark-set to achieve this (look how the developer of Downsize managed this ;-)).

Support: Support is very fast and reliable. Just had an request on Sunday!! and had an reply within a few hours. That's the way i like it.
Price I think the 20 bugs are a fair price for this software.

Overall Rating:I really like this software and i use it on a daily base. So if you're looking for a software with outstanding features for quick image processing: iWatermark is a good choice.