domjudge is fast and stable
Reliable and runs smoothly
This project has much more documentation than PC^2, and being open source there are no restrictions on its usage and it can be modified. Unlike other systems, users don't have to download any software, so in situations where computers are not provided participants can get started much more quickly. Among other things, the format for automatically adding test data is convenient and DOMjudge allows for multiple servers to prevent a crash from ruining competitions. The only negative things are that no one seems to use DOMjudge, so it was hard to find (I found out about it through Google), and it takes a bit of time to configure, particularly for Java.
My view is probably biased being one of the developers, but also multiple other users (teams, admins, judges) find this system way better than PC^2 and also better than Mooshak; these two systems are commonly used in ACM ICPC contests.