It's utterly wonderful. Really. I reviewed it in a recent HotPicks, and was thoroughly impressed by how they'd crafted everything
just right. You don't even need to install it -- you just execute any program supplied in
.package format, and it'll download the (small) Autopackage infrastructure.
That's the goal: no dependencies, no repositories, no
libfoobar.so.0.1.5a, no 'package managers' -- you just find an app, double-click it and you're done.
IMO, Autopackage's success could have a huge impact on desktop Linux. Despite what some people say, repositories and apt-get aren't a solution; they work adequately in the short-term, but once you need an app that's not provided, you're stuck. Compile? Takes time, and may need more dependencies. Grab another binary package? Could be problematic if it's not made for your distro. Upgrade to some 'testing' distro release? Why should anyone have to for a new app?
AutoPackage ditches this whole mess and, most crucially, avoids the horrible duplicated effort going on amongst the hundreds of distros. Right now, there are more people building, patching, rebuilding and repatching some programs than there are
developers for that program! Instead of every distro having to patch, build and package big software, there can be one source: the .package.
Some folks have argued that it makes software installation a bit Windows-esque. Is that a bad thing? There's no point in being different just for the sake of it, when it means Joe User can't run the latest FooApp because his distro repository doesn't have it... and to compile from source he'd need 15 other -devel tarballs... and there's a dodgy package in an 'experimental' repository which clashes with another app... and and and... *sigh*
This flummoxes newcomers who're accustomed to the ease of downloading a .exe, double-clicking it and using it. That's no great Windows innovation -- it's how many OSes have worked for years -- so in 2005 it's about time Linux got to the same stage, methinks.
OK, rant over

(Well, until someone else mentions Autopackage. In a nutshell: it's excellent, and I absolutely pray it becomes dominant.)
M