I want to adopt the hell out of these, but can't even gauge when/if I should. In my opinion you should be able to open any excel document in both apps so long as they are both using only cell functions because the alternative is very confusing for average users, so this stops me from using them except for pet cases.
@Joe McDaid Great to hear! I suppose the strangeness I am experiencing has to do with me being on the "targeted" release ring. I put myself in it within my tenant (without putting the whole tenant in the targeted ring) to test new features before they hit the general group, but maybe excel online is not in sync with my desktop apps "new features". Not sure how the online experience schedules into the Microsoft release rings. I notice SharePoint definitely seems to get targeted updates that other users do not have, I was expecting the office web apps to follow the same sort of schedule. Greatly looking forwards to GA.