I've never played In Nomine,but I've DM'd it many times and then abandoned it
Its a fascinating game to read, the background is awesome and I loved the mechanic ( the d666 system) but as Teflon Billy said it is a very difficult game to DM effectively or well and it can be to play.
You can play it as Billy said as "secret agents gone native" with angels/demons being pretty close to human, but I think if you play it straight as the game is written, celestials are much more primal beings than humans they are much closer bound to the fabric of creation and have much less free will.
An angel continually walks a tightrope between being true to his Resonance ( nature) and acquiring Dissonance and ultimately Falling ( unless of course you're a Malakim).
Demons also, are by their very nature quintessentially evil, they have few if any redeeming features - if they did they'd be on the slippery slope to Redemption.
It can be difficult to roleplay unrelenting evil, its much more than exploring the "dark side of gaming", and even more difficult to DM. The borders are much less blurred than they are by the alignment system in D&D. Celestials must pretty much act according to their nature or pay the price.
Still, enjoy the game, most of my players really enjoyed it( and still occasionally badger me to run it), I just ultimately found it a pain to DM.