I have been checking and can't find any information about what special abilities the improved familiars give to their master. Specifically, I would like to know where I can find out what an elemental familiar would give to it's master when it becomes a familiar. I have tried searching, and can find the expanded list for improved familiars, but it only shows the level needed to take that familiar. I click on the familiar type, if it has a link, and only get the bestiary entry, and there is nothing else that I can find. Please help, as it will make my decision easier for when the time comes to get my next feat. Thanks all!
Hm, sorry this answer is a bit late but here's what I've come up with....
In addition to their own special qualities, all familiars grant their masters the Alertness feat, the benefit of an empathic link, and the ability to share spells with the familiar.
As far as I can tell, the creature itself confers no other special bonus like a Toad giving you 3 extra HP or a Cat giving you +3 on Move Silently checks.