I play Eberron, and think its definitely the best bought setting i've seen for some time, mainly because it allows so many plot options for the DM, background options for the PC's, and allows new options without removing the classics. Plus it actually has 4 new races, and to be honest the warforged are probably the most boring of them. Shifters, Changelings and kalashtar all have amazing role-playing possibilities and the other races are all refreshed to the stage where even the gnomes are interesting! My favourtie bit though is the fact that there is a distinct flavour to each nation and race that doesn't feel like they've just copied a country from the real world (they might have taken ideas, but it doesn't feel like a Cut & paste which was a real criticism of some older settings)
I started with Races of Eberron (as a player) and moved into ECS. The ref has those, plus a Sharn sourcebook and a few other bits and pieces but i think a few ECS and Races in the group is all you need to get going.
Of course, i think my own homebrew is better, but what DM wouldn't....