It really depends on the clips you've seen.
So far there have been no "eye candy" clips from Bungie themselves. But if you grab some of the GC clips there's some really awesome stuff. There's a couple things to remember, like what you're actually looking at.
It's easy to say "looks like halo 2" because the characters haven't really changed (it starts a few days after halo 2, it'd be really odd if suddenly everyone was in totally different armor

) It's also difficult to compare directly with Gears or Killzone because the nature of the fights are different. Gears and Killzone have 4 guys against never more than 6-7 guys at a time in little narrow environments. Halo is 4 guys against dozens of enemies on huge open spaces with vehicles.
Keeping that in mind, go have a look at Gametrailers, some of the GC clips, and there's a really nice "omegathon" clip from PAX "t_halo3_pax07_omegathon_round1_gp_gt_h264.wmv"
It's about 120mb - the video quality isn't great but you can see the effects and some of the weapons. There's an awesome over the shoulder MP clip from GC over on IGN as well, have a look at this:
http://streamingmovies.ign.com/xbox360/article/814/814905/halo3_082207_multiplay_wmvhd.wmv
Look at the self shadowing on the weapons and vehicles, atmospheric particle effect, bloom, ambient lighting. Personally I think it looks awesome, but it's not trying to sell itself on graphics. The got me when they said the hornet will be available on Sandtrap . . .