Technically, 600mm is a little restricting... It'll give a unique look for sure, but it has it's drawbacks. As MattKing wrote, you will indeed have atmospheric issues with landscape photography using that sort of thing. I shoot panoramas with a DSLR and 300/2.8, so that's about 450mm equivalent, and air quality, visibilty, and temperature "shimmering" are the biggest things that affect image quality. Unless you are shooting baseball, I don't think there is much you couldn't do with a 300mm for compressing your composition. Here in Maine, depending on the time of year, probably 1 day in 10 is good for supertele landscape because of air issues. Winter is better, but summer is very iffy due to haze, smog, fog, heat, etc...
Artistically for non-landscape scenes especially, go too far with flatness your perspective and you're back in the dark ages of art prior to perspective, or you're like a 5 year old drawing figures. It's nice to be able to dispense with perspective once in a while, but it's easy to go too far. Small DOF also goes with it. Eliminate all foreground, background, perspective, and it's SOMETIMES a copout for thinking about those things. Yeh, it can make a nice image sometimes, but the context, storytelling, and creative possibilities can suffer in the process.