I got this demo at the weekend just there, and I have to say I really enjoyed it. I've been followig
DI with interest, downloading the developers diaries and such, and have been really looking forward to it.
Admittedly, I was a tad concerned with the start of the game (the whole Grim-Reaper battle) but was pleasantly surprised with everything after. Death by eyeball + crucifix merging, CHECK! Gratuitous boobage and spiritual intercourse, CHECK! Jesus-laser, DOUBLE-CHECK! Portrayal of a Bishop as a psychotic crazyman, CHECK!
ONce you start to acrue powers, the gameplay improves substantially, although as you say, it is a blatant GoW ripoff (if we want to get really technical, it is more akin to the Devil May Cry games, much like GoW before it)
However, the subject matter just totally hooks me, and combined with the fun hack 'n' slashery of it all, I really enjoyed the demo

a definite purchase for me.
And I repeat again - JESUS-LASER!!!!!