Yeah, not all apps support landscape, but the important ones do. E-mail, messaging, web browser and some of the third-party apps I've tried do support it.
There's some strange behavior, though. For example, you can search the marketplace in landscape but then the results list comes up in portrait. It's not a dealbreaker for me, because since I'm done searching I just close the keyboard and reorient. And, as I said, anything that requires extensive typing supports that format.
Having arrow keys is nice, moving a cursor with the touchscreen is usually an experiment. My mom has the Focus, and I've noticed that the on-screen keyboard, especially in landscape, is quite usuable. But I like the convenience of the keyboard and not having to dirty up the screen so much.
I used to want the Venue Pro, but watching WP7 news on twitter I see complaints about that phone more often. I would read some reviews, I think some of them had said the keyboard felt a bit cramped. Naturally a landscape orientation gives you more room. So, LG's not a bad choice. You'd really be trading the bigger screen (4 vs. 3.5) for a bigger keyboard.