I am a bit of a spreadsheet junkie and have tried most of the spreadsheet apps for the iPhone platform. Of all of them I find Mariner Calc to be the most useful. It is relatively straightforward to create quick spreadsheets for on the go calculations in the field, or simple off the cuff databases. Its supply of functions is reasonable given the limitations of the iPhone platform, and I have not had difficulties with spreadsheet transfers between phone and desktop. I keep this app on the first page of my launcher.
Some of the difficulties mention in earlier reviews may be due to the interface. For example, the formatting settings stick fine on my iPhone - once I learned to press the Apply button before the Done button in the formatting menu. The iPhone is a different sort of platform (for me at least) and each new app seems to have its own quirks. Mariner Calc is, however, quite intuitive to use in my opinion.
jwcman about Mariner Calc Spreadsheet for iPhone