I bought and use both the iPod App and the matching Mac application. Both work very well. The only thing I dislike is having to sync it through wifi. I still bought it though because I am confident that if/when Apple allows applications to do syncs through iTunes Cultured Code will quickly update the software to use the feature.
Personally I've used both since they were close to when they first released and I find that it was a to do list for me that just scaled. I didn't have to think about it - it just worked. It's helped keep me more organized and more importantly a place I can throw things to do into that aren't important now so I can think about them another time without trying to remember them.
I purchased the iPhone app 2 days ago. I've found it very useful as it has already helped me get 6 tasks done that I had written down weeks ago. Highly recommended. Now I have to decide if I should buy the Mac version as well.