We lost one alien game last night, dropped SC first and didn't protect the second hive. Thats where we failed.
Game dragged on for a good long while after that - marines teched up, still tried to siege the hive, and ended up finally walking in to finish it.
Pretty poor, if you ask me. I think the problem lies with marines still living in the last beta, where sieges were king. If you use a siege strat now, then you need to pray that the aliens slip up somewhere.