AHAH-powered forms were virtually impossible in Drupal 5 (see the note though). In Drupal 6, this is much easier, thanks to the #ahah property. However, it still is really painful to actually use it.
The flaw {#flaw}
You have to write a menu callback for each AHAH-enabled form item of your form. You have to repeat small variations of this piece of code for each callback: