As for Ava, I really hope they get rid of her soon. Desperation really isn't an attractive quality, and with each time she returns to see Alex, she becomes more and more.. tragic, I guess, or pathetic. She knew that much of the time she was there, he was attracted to Addison. She's asked him for a reason to stay twice now, and he's either failed to come after her or failed to put in enough effort to find her. If he really felt that much for her, and wanted her badly enough, he would try. Instead, he's moved on very quickly, into a relationship in which the success or failure would have more of an impact on his personal life, since Lexie is Meredith's sister/a coworker. He may have claimed to miss Ava in the premiere (although based on the relative levels of chemistry last season, I felt like him missing Addison would have made more sense, but anyway), but if he missed her that much, why wouldn't he either try to get it back, or use the opportunity he was recently given? Her advances have been met with a healthy dose of apathy-- I just can't imagine going back for more.
I think that Alex and Lexie have some shared experiences that will make it easier for the two to identify with each other. I kind of like them, so far at least.