I still use Day One…just. I was grandfathered with a “Plus” account but after horrible recent sync problems I deleted my account. I’m now using Day One on my Mac more or less exclusively.
I export regularly as markdown (which works well) and import into DEVONThink. Pretty much the entire procedure is automated by Hazel and a DEVONThink smart rule. I then use AppleScript within DEVONThink to replace any Day One links (i.e., cross-references to other journal entries) with DEVONThink links and to convert (manual—because Day One tags don’t export when using markdown export) hashtags to hidden markdown metadata (which displays the tags in DEVONThink but removes the original ugly hashtags).
The joy of all of that is that I’ve added many more photos to my journal entries in DEVONThink than I ever did in Day One (because of fear of encumbering sync) and (above all) the infinitely better search facility in DEVONThink than the paltry, slow search offering in Day One. (I should perhaps say that I have 18,000+ individual journal entries. Searching those in DEVONThink is sheer pleasure!)
Stephen