I think Halo was a OK shoot em up game. Not much fantastic in that game if you compare it to the shoot em ups that were already around on the PC at that time. So not much new stuff there.
For a console game, it was one of the best shooters of that time I guess. But still as a PC gamer I was not impressed with Halo.
Halo 2 looks like the same stuff imo, with some new moves in it, new story, better graphics, new enemies, new environment and that's it. Nothing really revolutionary there.
The shooters that will probably rock however are Doom 3 and Half Life 2.
Also Killzone for Playstation 2 looks very interesting.