GLaDOS is a psychopathic monster, and *always* intends to kill you. If you don't

>! escape the incinerator, or you fail to shut her down during the neurotoxin sequence, you die. She never has any intent to give you the cake or the party - these are just things she uses to entice you.

At the end, you can see that she *could* have made good on her promises, even though there is no outcome in the game where she *does.*

With regards to the cube, however (Portal 2 spoiler):

>! She really does have your cube, and she returns it to you in Portal 2 when she ejects you from the testing facility.