The only thing I regretted when purchasing my 13" rMBP last year was that it didn't come with 16GBytes of RAM at the price point I could afford. With a few apps open the 8GBytes it did come with gets almost completely used up! I didn't know Mac's were such memory hogs! Being a former Windows user that is (who's glad he switched anyway).
My experience with Siri is that it's still very flaky and for me anyway Google Now still works much better. I'm glad to read here that it's improving though. But it's going to have to get a lot better before I go back to it.