Just read this...truly a brilliant series.
Its basically the same premise as Marvel's "Marvels" series - we follow an ordinary joe (this time a cop rather than a photographer) and his life as he witnesses the birth of the Golden Age of heroes, then the Silver Age and so on and so forth - the history of superheroes unraveling in parallel with his own life.
Legacies somehow doesn't quiet capture the magic Marvels did but it does a thoroughly good job giving us snapshots of the various eras of the DCU. The earlier issues, recapping the Golden Age and Silver Age in particular are marvelous. With the later issues, things understandably tend to get cluttered a bit (given how much status quo's have changed in the Modern Age and how 'event-heavy' contemporary comics are). Also the choice of events to focus on gets a bit odd sometimes - stuff like Crisis, Death of Superman, Knightfall etc. are totally understandable but an awful lot of time is spent on Final Night and Hal Jordan's rebirth as Spectre. Also the history for some reason ends rather abruptly just before Infinite Crisis - not at all touching upon the last half-decade of pre-Flashpoint continuity.
Time works in a rather wonky fashion in this series too (hardly any time seems to pass between the Golden Age and Silver Age - but the time from Superman's debut to Infinite Crisis seems to span around 25-30 years - with our everyman audience surrogate and his family ageing but the heroes - not).
But on the whole its a great little exploration of DC's history and some rather obscure corners of the DCU in the fun little 'Snapshots' backups.