Have been playing the Halo 3 Beta through Crackdown. I still take issue with the small field of view and the beta in general feels more like a well polished demo that has been released after the game was released and then patched. It is way more polished than any beta I have ever played before, but the weapon balancing, map navigation, weapon placement on maps, sound and graphics are all better than on any previous Halo so it's a thumbs up from me, but something that stands out that you might not have heard about is the FileShare feature.
FileShare is this thing where after you have played a match you can choose to save a replay of the match to memory (press Back and then X) so you can then watch it back whenever you like, but Bungie gives you some server space to go with your Halo Profile (24MB of, with videos taking up anywhere between 2MB to 6MB of), this allows you to upload replays which can then be downloaded by anybody and watched as many times they like. So if you played against someone very good, you can check their FileShare, download a video or two and see how they play, possibly even learn from their videos.
The thing I like about this the most however is that while running Halo 3 you can hit the GuideButton, select a friend or someone you recently played with and at the top of the GamerTag blade is shortcuts to their Halo medal case and their FileShare, which makes it ultra easy to find videos of people you have played with or friends. But better still is if you play with someone you know, try exchanging recordings of the match you just played together and then watch each other's videos at the same time. That way you can both learn from each others play styles.
Plus with the FileShare I imagine we will be seeing far more Halo related videos uploaded to YouTube as all matches are being recorded now as you play. If something cool or unexpected happens you don't have to grab a camera, just save the video after the match. Here is one I already found titled "Obie Vs The Warthog From Hell". This video quite accurately portrays some of the difficulties I myself have encountered with 1337 (and average) players.