The class also casts arcane AND divine spells, allowing for a stupid array of accessible prestige classes. Divine spells, however, do take hours to actually prepare. Though, a gen can grab that spell while a sha'ir is asleep.
Spells known progresses at the same rate as a sorcerer, but the sha'ir has twice as many spells. Probably a good idea to keep every spell you want access to all the time right there. Spells with super long durations or super narrow applications are probably fine left out of there.
Diplomacy is a class skill and is integral to actually getting spells. A sha'ir has a bonus to spell negotiations equal to its class level. With full ranks in diplomancy in addition to that, getting spells is simple. Early on, not so much. The diplomacy DC for grabbing a first level spell that is 22. A first level sha'ir will probably have a +9 (4 ranks+4 cha+1 level) to the check, with an additional +1 or 2 depending on race and another +2 for having that spell on the spells known list. A level 2 sha'ir would have a +15 (5 ranks +4 cha+2 level+4 synergy(bluff/sense motive)). Beyond level 3, the check becomes increasingly trivial with a few failures here and there.
At level 18, a sha'ir planeshifts at will. This makes for a wonderful attack that sends enemies to any of the elemental plane of so desired.
oh god skill check for bonus spells.... Item familiars ahoy.