The increase of storage is actually in my view more important than it may seem. I had a Google Business account that was free until recently. I switched to a paid Proton account as a result. The fact that it was free was the only reason I stayed there. As Proton had only 5GB at the time and my inbox at Google was much larger I chose to just backup all the old Gmail stuff and start fresh on Proton. Had it been 15 GB as it is now I could simply have used the very simple migration option in Proton and had it copy over all my email. So for people switching now, it is much better (if you like me keep way too many old emails...
Experience so far: Proton works well. I do value the added security and most of all privacy. But it is a bit rough around the edges. Spam is worse on proton. Not very bad, but I do get a lot more that Google was able to filter. The apps and webinterface are fine but there are minor annoyances. For instance if I read an email on one device, on the others the notification doesn't realise this. So when all my mail is already read i.e. on the webinterface, my phone and tablet still have the notifications on.
There are other small things, but all in all I'm happy with the move.