I've recently tried the fish shell and I'm loving it. It really seems to fit with the Ubuntu philosophy. Making the shell easier for newbies is a huge plus since it can be pretty daunting. Fish isn't entirely POSIX complient but only breaks compatibility if they can improve upon it. So some shell scripts may not work but the interactive experience with the shell is wonderful.
I think work is being done to include dash as the default non-interactive shell since it is faster than bash (and maybe fish). Rafaël Carré
Fish's real benefit is in interactive mode. Would it be possible to make dash the non-interactive shell and fish the interactive one? CoreyWoodworth
fish is amazing and dash is fast but it's interactive mode is scarry. I think Ubuntu would benefit alot by combining fish and dash. Their should be away to do that ChrisWoodall
I'm not sure you can have all the functions of Fish, plus Bash compatibility, and have it run as fast as dash. Speed isn't really the important thing about Fish. ChrisLees
"So some shell scripts may not work but the interactive experience with the shell is wonderful."
Aren't scripts interpreted by whatever shell is specified at the beginning anyway? If it says #!/bin/sh, then it doesn't matter what your default shell is. If we can make life easier on users by changing the default shell, we should definitely do it. I've been using fish for several months and it's definitely an improvement. Are there better alternatives? Hotwire shell looks interesting, but not sure how good it actually is. Endolith