Having been a philaformuser on Joomla 1.0 it was hard finding a 1.5 alternative. RSForm!Pro is our tools of choice due to its incredible flexibility. You can catch the processor before and after it submits and write your own code into the process.You can fill out form elements based on query strings padded in url. There some tips and tricks to use with it but I have yet to find something it cant do. The template system is also great. You can customise your forms radically to give them distinct looks or use the built in templates for 1 or 2 column layout. You can use it as a simple contact form or a campaign petition tool.
Love it love it love it!