The way I look at it is, is it worth a feat? You can always get Obtain Familiar. That's even a better familiar that way, since nonwizard levels generally don't advance familiars. But yeah, it is a tradeoff. But you gain an ability nobody else in the game has.
Another dirty trick to keep in mind is that being a prohibited school doesn't actually stop you from *preparing* spells, just putting them into your spellbook. You can also put spells into your spellbook before you can cast them, provided you find the appropriate scroll and can make the spellcraft check. So pre-load on any spells from your incantatrix school.
Finally, look up the Otyugh Hole. It is a magical location in comp scoundrel. Spend a week in in and get Iron Will free. Costs 3k, but totally worth it.