I'm pretty sure that noone uses a familiar for anything save as a boost of hp for the mage. I wonder if anyone found a good use for the little critters outside backpack?
The fairy dragon makes for a good spy, wich is kind of worthless after one have gone trough the game once, but is also a good way to get you some time to prepare if faced with a sudden ambush. The invisiblity 10' radius is fired pretty darn quick and gives to to regroup and prepare. Also good for dealing with them pesky mages for a little while atleast.
This is the only familiar I have used so far so I do not really know what so say about the others yet. The Dust Mephit has the best dialogue though. I have no idea why they choose to make the neutral familiars so darn unlovable. I thought that the familiar was supposed to consider the mage their best friend (and vice versa).
It would be really really cool to have Cromwell (or Jan Jansen for that matter) to be able to modify items so that the familiars could use them (pseudodragon using Jans unique crossbow would be really neat. "Say hello to my little friend" ;-). Just being able to use wands or scrolls would make the familiars infinitly more useful than they are now.
Not to mention that they could have added the familiar boosting spells from Dragon Magazine.