I don't do too many landscapes at the moment but I used to and I do own a Panny 7-14mm and it would be fabulous for this. I assume the new Oly in this range would also be very good. The Panasonic does't work as well on Olympus bodies, so it depends on which camera you own.

I have the 7-14 and it is great. The F/4 isn't perfectly ideal, but it is constant which is a huge step up. In most cases, compensation with ISO works fine.

I did a lot of research prior to buying it and decided not to wait for the Olympus equivalent due to the much higher price.

Here is a large thread on it:

http://www.dpreview.com/forums/thread/3783188#forum-post-55096344

There are no real other contenders as far as I know. For super wide, these are the two choices.

Another tip for you (and anyone else who is looking at/using one of these) is if you want a graduating filter solution to go with your lens, which you will need 30% of the time when taking landscapes, is to get this:

http://bhmstore.com/774-2055-thickbox/cokin-p-series-universal-adapter-ring-p499.jpg

Along with the associated cokin filter holder (you can get a knock off from ebay for cheap) and then a really good graduating filter, you have a full filtered super wide solution.

There is very slight vignetting with this solution, but is quite negligible with a little post production fixing. In fact, I doubt there is a good way to filter this lens without some vignetting without actually cutting the built in hood:

http://www.mu-43.com/threads/76442/

I have the Olympus E-M1 and the 12-40mm f2.8 pro lens. Everything about this combo is amazing. The 12-40 is a wonderful landscape lens. Although it's not the widest lens available, I've found it easy to produce excellent stitched panoramas.

The 40-150mm f2.8 pro lens is also stellar. I used it during a shoot in Costa Rica and the feather detail on the hummingbirds and other birds blew me away. The 40-150mm also performs well with the 1.4 teleconverter, which I used quite a bit on that shoot. (The lens is, unfortunately, a bit short for a lot of wildlife shooting.)

Both lenses have a constant aperture throughout the zoom range.

I'm very happy I switched to the Olympus mirrorless system; everything's about half the weight and size of traditional DSLRs and the IQ is superb.

Edited to add: should mention that the E-M1 has in-body stabilization so the lenses are not IS. This makes them less ideal for people who have Panasonic bodies. The 5-axis IBIS is pretty terrific though; I can shoot handheld at much lower speeds than I could with my previous gear.

Thanks. It's nice to have comments based on real life experience.