I actually find Spielberg became a better and more mature filmmaker over the years. The craft and masterful storytelling between 1998 and 2006 ("Saving Private Ryan," "Munich," "Minority Report," "Lincoln," "AI," "Catch Me If You Can" and "War of the Worlds") far exceeds his, sometimes, stumbling work from the '70s and '80s (although I do love "Close Encounters," "Raiders of the Lost Ark," "Jaws," "The Sugarland Express" and "Duel"). I despise with a passion "The Color Purple," "Empire of the Sun," "1941" and "Hook." If anything, I'd say the recurrent flaw with Spielberg is that he can never close the deal in a movie, his endings always fall short. He always overreaches. Also, since 2008 and his failed attempt to revive Indiana Jones he has been very inconsistent and can't seem to find a groove with his films (with the exception of "Lincoln.") With all that being said my favorite film of his will always be "Schindler's List," I just can't see him ever topping that film.
Now to answer the "never-ending" question, is he an auteur? There is something to be said by the mere fact we can discuss Spielberg as an auteur, although his themes are fairly difficult to pinpoint. His themes on universal goodness, and the fantastic existing in reality if you know where to find it, are more obvious in his harshest movies (Saving Private Ryan, Amistad, A.I.) and find their way into his technique: whatever the moral (or lack thereof) a lot of people like them, and watch them, over and over again, because they're unassuming. His use of the medium can be impressive - impressive enough that the majority of people don't think about it, and they can enjoy his films regardless.
His technique looks simple, yet it's incredibly effective. When I re-watch his classics, the way he edits, shoots, and lets scenes breathe is pretty fascinating. He loves his long shots. Not many directors gets to achieve such mainstream success and then have their movies studied over and over again, by countless individuals. Now for the ultimate auteur theory test: Would I know a film was directed by Steven Spielberg if I didn't know anything about it (including who its director was) going in? You bet your ass I would. The daddy issues, the streams of light, the John Williams scores and, of course, there's these recurrent shots he keeps using (HERE).