Have saved games been compatible after patches?

Yes. Factorio has historically allowed saved games to be migrated between versions, and the 0.14 to 0.15 migration is no exception. Factorio 0.15 will be able to load any save that was made in Factorio 0.12 or later -- from the patch notes:

  • Disabled loading of saves before 0.12.0 version (You can use 0.12 to load older saves and re-save them).

Generally speaking:

  • Terrain will be preserved. If the game's terrain generation changed significantly between versions, there might be discontinuities between old and new terrain, but I don't think there's any such changes in 0.15.

  • Some ore patches will probably be created for the new uranium ore.

  • Most structures and items will be preserved. Some buildings which changed sizes between versions (like the fluid pump) may need adjustments, and I'm not sure what will happen with items that have been removed in 0.15 (like alien artifacts and science).

  • Any research you have completed will still be researched. You may need to research some new technologies if new prerequisites were added.