It may not be too soon to say that Jeb Bush will be the Republican nominee in 2016. I'd say he is now the odds-on favourite.
Romney has pulled out. He saw little breathing room for his candidacy. That means the Republican establishment is closing ranks. They have made their choice, on the traditional basis: it's Bush's turn. Romney has “had his chance,” and Bush is old enough that he is unlikely to get another. Other candidates are young enough that they will.
The Republican establishment, the professionals, almost always get their way in the end.
Christie will probably stay in as an establishment backup in case Bush stumbles. Huckabee will probably hold down the Christian right, Ted Cruz or perhaps Rick Perry the Tea Party, Rand Paul the Libertarian wing, assuming they all go in, all able to make good showings. But with the planned big bang in the South very early on, there probably will not be time for the rest of the field to shake out quickly enough to leave a clear alternative to Bush, until the nomination is all but locked up.
In the meantime, word is that Hillary Clinton's numbers are starting to fall rapidly. This was predictable. In fact, I predicted it here. She was riding high on sheer name recognition, with people not yet focused on the race. Now that the race has tentatively begun, with Jeb Bush virtually in, people are beginning to pay attention and to look around.