Radeon HD 6950 - slightly overclocked - getting about 5-6 fps worst case. Goes up to 15-20 on some view angles (when there isn't much to raymarch). That's at 1920x1080 resolution. In 720p it's kind of playable.
I think raymarching has its uses but it's still too much work for real time rendering of large open worlds. There are some things that I think it could be good for (rendering clouds comes to mind).
If you are interested in procedural world generation I assume you've come across procworld, perhaps you could use something similar? It's voxel-based but there are some definite advantages of ray tracing over ray marching distance fields (determinism, spatial and temporal coherency, notably) though I'm sure both are useful.