I'm probably going to wait until I have a better idea of how well supported the Hard Drive will be.
... Since it is an optional component now, I'm guessing that it will only be supported by a few games a year. If the only other reason to get it is for backwards compatibility with my current Xbox games, I'll wait on the Xbox 360 for about two years.
Why?
I do not have a High Definition television. I actually... loathe most television programing, and we haven't used a TV as much more than a large console game monitor for six years now. I probably won't buy a HD TV until my current TV starts to break down.
I've /tried/ to enjoy playing Live over the last year and a half, and frankly... I hate multiplayer games. Because of the multitudes of /complete jerks/ out there, and the lack of cooperative games. Cooperative. Not team deathmatch. A team versus computer controlled and based objectives. I'd love to see more games like that, as people seem to act less... juvenile and annoying.
The Xbox we have now is the only console that my wife regularly actualy plays with as the Hard Drive makes saving and loading less of a hastle, and the load times are bareable for her.
So, as I do not have the capability to fully enjoy the visuals, do not have the mindset to enjoy the abuse that spews from Xbox Live, and cannot see how a neutered Xbox 360 will actually hold my wife's attention for long, I'm going to wait until a major pricedrop, and used cheap games are plentiful.
I know it's not a big deal for everyone, but hearing that the Hard Drive is an optional component pretty much killed my enthusiasm. I'll probably get a 360 eventually, just not in the forseeable future. It /is/ a good move by Microsoft, however, as far as business is concerned, and it makes me confident that there will still be a lot of competition in the console world.
Edited to add:
The saddest part of all this? My wife brought up the 360 tentatively to see if it was something I would enjoy as a birthday or christmas gift, and I had to explain that I had no interest in it at this point. Seeing the interest in her own eyes die as well was... rather sad.