The cake wasn't really a lie though, was it?
GLaDOS mentions: "The Enrichment Center is required to remind you that you will be baked, and then there will be cake.", and that is exactly what was her intention.
I only played and finished the game maybe a few months ago, so perhaps I was numbed by all the cake jokes (were there any actual jokes?), but the ending wasn't super special cake-wise.
The surprise after the final test chamber was nice, I liked finishing the game that way, but I never really "got" the cake.