For me, it's not compatible enough. At work we have a few batch scripts that set up the path and set some environment variables, so we can launch compiles/tests/etc with a short command. These work under cmd.exe, but not under powershell. I keep meaning to experiment with powershell, but never quite enough to overcome that initial barrier. (I'm 95% on Linux, so it's not as if improving my windows workflow would make a big difference.)
Turns out that the batch file thing is a fairly common issue.
If you install the Powershell Community Extensions (PSCX), a pretty useful Powershell module in general, it has a command (Invoke-BatchFile) to execute a batch file and propagate the environment changes back to powershell. (While this is not always sufficient; I have found it to be enough for most purposes)
If you're 95% on Linux, you might like PowerShell just because it's a bit more familiar. You can run 'ls' and 'rm -r somedir' just like you're used to (although with quirks, 'rm' only takes one argument, but the argument can be a wildcard... which is expanded by rm, not the shell). I'd say I spend 98-99% of my time in Linux or OS X + zsh, and during my 1-2% time on Windows I was using MSYS until I found PowerShell.
