Background
Setting the stage for the paper describing their work, Stevens et al. write that "ECHAM6 is the sixth generation of the atmospheric general circulation model ECHAM, developed by the Max Planck Institute for Meteorology (MPI-M) in Hamburg, Germany."

What was done
The eighteen collaborators primary goal in producing this paper was "to document, in the open literature, ECHAM6," and to assess its current status "in the context of over a quarter-century of development."

What was learned
Although Stevens et al. say that "ECHAM6 better simulates many aspects of the present climate as compared to ECHAM5," they acknowledge that "a number of stubborn biases endure." These biases are said by them to be the facts that (1) "there has been relatively little improvement in the representation of precipitation since the introduction of ECHAM3, (2) "precipitation over land is too weak," (3) "there is a global tendency of precipitation features, from the South Pacific Convergence Zone to precipitation over the Gulf Stream, to be shifted too far northward," (4) "biases in the representation of marine boundary layer clouds remain large," (5) "cloud layers appear too infrequently in the simulations," but are (6) "too bright when they do appear," (7) "ECHAM6 continues to have large (3 K) cold biases in upper tropospheric temperatures," (8) "tropical temperature biases only vanish with very high (300-m) vertical resolution in the upper troposphere," (9) "the tropical and mid-latitude convective stability of the troposphere remains more unstable relative to observations, particularly in the southern Hemisphere in its summer season," and (10) "the extra-tropical jets maximize at somewhat lower latitudes than observed."

What it means
With this many things wrong with it, perhaps the ECHAM6 climate model should not be called "new-and-improved." Reworked-but-still-not-there would seem to be more appropriate.