Each of my in-depth reviews addresses five aspects on online correspondence play that I consider essential.
1. Playing environment: board and interface style, functionality, and options; in-game conversation and planning; time controls; menus; support of chess rules (software recognizes checkmate, enforces draw offers after repetition, fifty-move rule, etc.).
2. Social environment: forums--chess and non-chess; leagues and teams; special features. The score here reflects the character and quality of the community of users as well as aspects of the site design.
3. Database functions: PGN download of games played by self and others; annotated games; database search capabilities. Only one site currently facilitates complete download of all my games, as well as all the games of my opponents. For many serious correspondence players, these features are critical.
4. Chess improvement: specific features designed to nurture improvement in chess skill.
5. Server: speed; downtime.
Below I rate all the sites I've played at for each of these features according to the following scale:
!! = exceptional
Spaces left blank indicate that I was unable to evaluate that feature with my level of membership. More detail regarding these assessments is available in the forums at Red Hot Pawn: http://www.redhotpawn.com/board/showthread.php?threadid=38770