As far as I know you are buying the full release of the game as a preorder, and the perk for that is access to the (now year old) Beta version.
I have very much enjoyed exploring the mechanics and small portion of the world which is fleshed out (both inside and outside the 'playable area'), and have been able to make nearly two dozen bug reports and feedback reports, some of which have been acknowledged as novel bugs that have then been passed on for fixing. It is harder to find novel bugs at this stage which haven't already been addressed internally, or previously reported, so the true purpose of the early Beta is not as important any more, but the access has been well worth the asking price for me so far (backer from the initial KS campaign, but only for Beta, not alpha access).
I gather the current asking price is a little higher than that given to the KS backers, for the same(ish) rewards.
However, I still would be happy if I bought into the Beta today, (with the acknowledgement that the Beta is buggy, slow, and feature incomplete (or at least inconsistent)). Don't buy it with the expectation of a thoroughly polished near release candidate Beta, as the only version of pre-release code will be the same Beta that was available one year ago, with all the warts and wrinkles. The internal development footage from the newer builds looks substantially more complete and refined however, so I have no concern about the final quality or the release window (2017) being met.
Even my ex-wife, who is no fan of computer games at all was impressed by the atmosphere of the Beta when I showed it to my daughter several months ago.
I don't recall any 'backer only' in game content, and think that would be a mistake anyway. Don't like that kind of thing really, and feel that all content should be available to all purchasers, regardless of when they made their purchase (and that early purchasers also shouldn't be price gouged on 'updates' to bring their version to the same as the standard retail versions available after the initial launch.
Whether you should buy in now or not is really down to you and your own decision. I generally don't pre-order, but did here because doing so back in early 2014 was the necessary support to get WH over the initial hurdle of demonstrating public interest in a non-fantasy, hard historical, first person rpg, which was not considered 'commercial' by most publishers. I would have found it a harder choice to make had I to decide today, as the project is near completion and the life of the Beta is no more than another 9 months or so, if not significantly less. It would be a 'day 1' purchase even if I didn't pre-order though, as it is very much right up my street, and I think I would still find it hard to not get in on if I were making this decision today. YMMV.