For the former, it's been pretty good with occasional crashing -- unfortunately, I haven't managed to figure out conditions for reproducing this reliably. It'd be nice for it to do previewing with interactive replacement -- may be it does, and I haven't figured this out.
For the quick-file-viewing, it happens to be fast enough for quick start up here, so it's what I have as a default viewer when using Coffee
Although a number of other editors with syntax highlighting start up comparably fast (if not faster, e.g. SciTE.exe), I don't think I've tried one that offers both of the above features.
It certainly would be nicer if Notepad++ offered out-of-the-box support for AutoHotkey!
For longer sessions I tend to use Emacs or Code::Blocks -- and if I happen to be using something like Cygwin or MSYS, I tend to use whatever comes up via vi.