Just found this thread. It's cool. I like 40k, and still have the White Dwarf where Rogue Trader made its debut. The lore has come a long way since then, but while so much has changed I do appreciate that the overall atmosphere of the setting has remained constant (forget the power of technology, science and common humanity. Forget the promise of progress and understanding, for there is no peace amongst the stars," - White Dwarf issue 93, September 1987) I think the setting can be taken seriously although it can also be very funny too. It has a very strong sense of satire, and the 'silly' side of it is more a product of a certain self-awareness than a consequence of the narrative. I never got into 40k for the tabletop in the 40mm scale although I used to play Epic, and later Battlefleet Gothic (in addition to Fantasy) which were both great, and incidentally less expensive than the standard game. Especially battlefleet Gothic.
I've dabbled in some of the 40k RPGs but for role-playing and story telling I have always favoured the fantasy setting especially the 2nd edition of WFRP, minus Storm of Chaos for the most part. Although 40k can be fun as well I think individuals can easily get lost in the setting. As that very first white dwarf issue said, 'But the universe is a big place and, whatever happens, you will not be missed...'