Huh, so it's just the 'Save as PDF' button that is not useful. I can print as PDF to save it to a cloud service. Weird, but functional sounding, I guess.
So, I just tried that out before posting this reply. It worked... some of the time. Sometimes I just get the "This printer isn't available right now", even after selecting the Save as PDF option from the menu in the Print dialog. Other times it does bring up a save dialog where I can select Google Drive (but no other cloud service). However, it doesn't use the subject of the email as a name. I have to enter that myself.
So, yeah - Google Drive only, when it wants to work, and I have to name every saved PDF myself :-( I'm sorry - I'm back off to Spark which makes all this a breeze.