May I beg my pardon in advance for a long post.
Let's get realistic about this, if you are a landscape photographer who cannot justify the price of medium format (and there are plenty out there), then the 5D MkIII is probably not the camera you've been waiting for. The 5D MkIII adds little value over the MkII for people who don't really use the AF, continuous drive or higher ISOs (I guess that the 100% viewfinder is nice). If you are a landscape-only photographer and don't already own a 5D MkII, then wait for the price drop and buy one, it will still be a great camera!
If you must have more megapixels, i.e. you think that you'll regularly be printing much larger than 13" x 19" (A3+), you'd be better off either switching to Nikon for the D800, or waiting a year or so to see if Canon follow up on their statement that they can easily make a hi-res competitor if there is demand for it.
For everyone else, the 5D MkIII looks to be a great all-rounder camera with a good balance of resolution and speed. [Of course there are subtleties such as buffer depth that could yet spoil the party for anyone thinking of using it for semi-serious action photography].
I think that it's interesting how we got to this situation with the D800 and the 5D MkIII; far from our usual accusations that our favourite manufacturer "just isn't listening to users", I think that these two cameras show they have. 5D MkII owners have been saying "great camera, enough resolution, but a poor autofocus system and it would be nice to have lower noise levels at high ISO". D700 owners have been saying "great camera, love the build, af and it's fast enough for my needs, but I want more megapixels [despite defending Nikon to the hilt in front of my 5D MkII owning friends]". Both calls have been answered. Of course this leaves a sizable minority without the feature they most wanted, just read Nikon Rumours to see those that wanted a faster D700 replacement with a low noise optimised sensor. But I think that the vast majority are going to be very happy with what is on offer in their chosen brand (barring a few minor quibbles that will be blown out of all proportion), look how many people here are excited over these rumoured 5D MkIII specifications. Of course, from our point of view, the "leaked" specifications still have to come good tomorrow!
To put it in perspective, the 5D MkII was such a bloody good camera that people bought it despite of its faults, now it looks as if all of these have been fixed and then some. Good enough for me...
