I've been a few times, I love Ibiza town.
not really into San Antonio though I have to say... there are many nice resorts which are a little quiter... one resort which I liked was playa d'en bossa, you have a great beach there and a fab range of bars and restaurants.
I've been there with just friends and done the Manumission partying etc which is highly recommended if no kids but also done it with the family too and my lo's loved it!