To start with I found the game surprisingly frustrating. I was playing on easy mode, but it was anything but straightforward. I was killed at least half a dozen times in the first level and found that the available weapons were not really powerful enough to take out the sturdy Covenant Brutes with any level of efficiency, leaving me hiding behind rocks to recharge my shield quite often. This was not the easing into the game I had hoped for.
The second level nearly made me give up entirely. It was a little easier (or perhaps I'd become more used to the style of play required), but this level showed up another major shortcoming in the game design, namely navigation. One mission objective asked me to go to a hangar to clear out some Covenant invaders, but neglected to tell me how to get there. I hung around in the command centre to see if any of the marines hanging around would show me the way, but no help was forthcoming - in fact I kept getting told "Chief, get to the hangar NOW!" After some aimless wandering around, a directional arrow appeared in the HUD which showed me the way. After clearing the hangar, a marine helpfully said "Follow me!" to the next objective. Where was he when I needed him earlier?
Frustrations were put aside on the next level as I got to drive like a lunatic in a Warthog running over the very same guys who had been wiping me out minutes previously. Most satisfying. This reminded me that my favourite bits of the earlier games were the vehicle sections. As Halo 3 progressed, it showed the same pattern of difficult on foot sections, and really cool driving and flying sections where I was kicking bottom! More cool stuff followed, such as taking out a giant Scarab tank.
Overall the difficulty level peaked in odd places throughout the game - the Flood being a particular problem due to their numbers and resistance to the weedy guns I could use. Occasionally I was able to grab a huge plasma cannon for a little while which was fun.
Plot wise the game certainly made more sense than Halo 2, with clear progression throughout. The ending was fairly satisfying, though the post credits "teaser" left things wide open for more (not counting the Halo 2 timeframe Halo:ODST). It would have been nice to have an optional "The story so far..." option to watch at the start of the game though. I occasionally found it tricky to hear what characters were saying over the bombastic soundtrack though - some of us have to play at night without waking up a wife and child!).
Overall, Halo 3 is definitely a game where (for me) the fun bits are the easier sections where you do cool stuff, and the harder stuff is just frustrating rather than challenging. I don't feel much temptation to replay the game on a higher difficulty level (even though most of the achievements can't be unlocked in easy mode, another punishing feature). It just feels like this is a pure hardcore game, which totally goes against its mass market advertising and sales figures. I can imagine a lot of people got fed up on the first couple of levels and gave up with it, which is such a shame for a game with such obvious love and attention spent on it.