In the course of our MAMBO 1.2 mm survey of the COSMOS field we have found a sample of 25 bright SMGs. While most of these sources appear to fit well within the scenario of typical massive luminous bright star-forming galaxies at typical redshifts of ~ 2, some of them appear to be rare objects either in the transition between a starburst to a QSO phase, at very high redshift given their lack of radio counterparts or embedded in substantial overdensities of massive galaxies at z~1.5-2.0. In this talk, I will present IRAM 30m, PdBI and SMA observations that point to this very varied nature of some of the most massive systems in the early Universe.