Mercurial is:

Fast
  • Extremely high-performance delta-compressed storage scheme
  • Optimized for disk layout and access efficiency
  • Complete cross-indexing of files and changesets
  • Bandwidth and CPU efficient HTTP and SSH sync protocols
Scalable
  • Distributed development model supports unlimited numbers of developers
  • Allows arbitrary merging between developer branches
  • Doesn't significantly degrade with large numbers of files or changesets
  • No waiting for locks!
Robust
  • SHA1 integrity checking on repository data
  • Append-only storage model with transaction journalling
  • Fast full-repository verification
  • Convenient backup
Easy to use
  • Most commands are familiar to users of CVS and other systems
  • Built-in command help
  • Integrated stand-alone web interface
  • Works with various GUI tools
Easy to adopt
  • Runs on UNIX, MacOS X, and Windows
  • Conversion tools available for many popular SCMs
  • Allows a variety of usage models
  • Supports user-defined hooks and extensions
Free
  • Source code available under the GPL license
  • Actively community supported and developed