[RedDwarf]

Guys, ZYpp is based on this: http://pho.ucsd.edu/rjhala/papers/opium.pdf

From the abstract:

Using off-the-shelf SAT solvers, pseudo-boolean solvers, and Integer Linear Programming solvers, we have developed a new package-management tool, called Opium, that improves on current tools in two ways: (1) Opium is complete, in that if there is a solution, Opium is guaranteed to find it, and (2) Opium can optimize a user-provided objective function, which could for example state that smaller packages should be preferred over larger ones.
We performed a comparative study of our tool against Debian’s apt-get on 600 traces of real-world package installations. We show that Opium runs fast enough to be usable, and that its completeness and optimality guarantees provides concrete benefits to end users.

I'm sorry for the others... but ZYpp is perfect. While

These numbers show that apt-get fails to find a solution when one exists in about 0.61% of install attempts.

Then you have:
- Support for metalinks (Duncan Mac-Vicar P. » Blog Archive » libZYpp, torrents and metalinks)
- "Services" (Duncan Mac-Vicar P. » Blog Archive » Introducing ZYpp services)
- Packages can depend of hardware, so when you install a new hardware the needed packages (drivers) to use it are installed automatically: Duncan Mac-Vicar P. » Blog Archive » Extremely easy driver installation
- Support for delta RPMs.
- Very efficient (Duncan Mac-Vicar P. » Blog Archive » yum and ZYpp speed / memory usage)

Why someone would want to use something different?