the Craig trilogy was IMO a sort of silent silent reboot of the Bond franchise, it brought it back to the roots but in a more gritty modern world, they were all about Bond becoming Bond and not a continuation of the previous films.
Casino Royale was all about Bond getting his 00-status and falling in love with Vesper and then losing her, it also (re)introduced Felix.
Quantum was partly fluff but very important from a storyline perspective in the way that it brought Bond into a more believable modern world and Bond moving beyond the loss of Vesper, it also developed on Bond's friendship with Felix, gave us a throwback with a reimagined Mathis, overall it was about modernizing the Bond world and the enemies but most importantly it was about character development and Bond earning his place in MI6 and the trust of M.
Skyfall was excellent if you ask me, it showed us Bond's past, something none of the other Bond movies ever did and neither did the books in such detail, the story was primarily about the past haunting the present (and the DB5 was a nice touch if you ask me) and Bond becoming the Bond we know from the other movies (a grittier version though). It reintroduced Characters, some reimagined, some very much in tune with the Bond movies of yesteryear.
They should be viewed not as separate movies but as one storyline (Quantum was just a continuation of the events in Casino), from here on the franchise can continue with business as usual.