I don't know anything about "The Last Days" as I never had that addon. I did purchase "Battle Over Europe" through Just Flight many years ago when it was new. Unfortunately, the last time I checked Just Flight's website BoE was discontinued.
Battle Over Europe, The Last Days and Ost Front are expansions for the dynamic campaign generator that comes with the game. If you like the stock dynamic campaigns, well, these addons are more of the same. In the case of BoE and Ost Front, however, the developers partnered with Maddox Games and constructed maps and objects to be included in Forgotten Battles patches, then sold the campaign addons as private enterprises. That is why the stock system doesn't have a U.S. or Luftwaffe dynamic career for the D-Day and Ardennes maps for example.
Is BoE worth the money? I'd say that the value diminished over the years with the plethora of user-made static campaigns, but the dynamic campaigns are certainly an important missing piece of the IL-2 FB/1946 legacy. If you have more desire to fly P-38s and Spitfires over Caen and Bastogne than you do for flying Yaks and Lavochkins over Moscow or Hellcats and Zeros off of carriers somewhere in the Pacific then yes, BoE is worthwhile...if you can find it.
Remove before flight