The way Halo 2 should have been....but it's still not utilizing all the 360's power.
CAMPAIGN
I played the game on Heroic and found that it was quite challenging--much more so than H2. The challenge came in numbers as I found myself often killed by Grunts and Jackals--a testament to the difficulty increase. However I was able to complete most of the game on my own. Arguably the hardest level in the entire series is a flood-infested level onboard the High Charity. I honestly did not attempt this level on solo, so cannot comment on it other than this: with 1 friend in co-op, it took us several hours and careful planning, ammo management, and luck to get through it. The game is that hard.
The plot is solid and ends just the way I wanted--both in plot and in giving a sense of finality to the trilogy.
MULTIPLAYER
Imagine Bungie taking every last piece of feedback we've complained about for the last 5 years and they implemented fixes for H3. That's how I feel. The new navigation/party system feels great. Here are some features that I really enjoyed: ability to majority veto a game/map, ability to EASILY partner up at the end of the match, ability to QUICKLY access a player's rank and stats, a SOLID SELECTION of both ranked and social playlists, etc.
So far my friends (ranks range from 30-50) say they've yet to encounter bugs, glitches, or cheaters. That's a massive plus. Hopefully things will stay that way.
So far every map I've played, I've enjoyed. Maps rarely feel too big or too small, which is a plus.
WEAPONS
Overall, everything is harder to use (and less powerful). The sword, for example, is tedious and difficult. It's harder to get a lock and easier to screw up a lunge. Weapons feel much more balanced--I've never picked up a weapon I wasn't able to kill someone with (including the needler).
Multiplayer seems balanced and you start matches with the assault rifle, which is heads-n-shoulders improved over the SMG from H2. Campaign has the new flamethrower, but it is really only useful against the flood--but in that situation it's priceless!
OVERALL
The game is completely solid and it is a great game. However, it is really not all that impressive for a next-gen console. The graphics are nice, but did not inspire me the way that Gears of War did in the day. They could have been better.