Anyway, as to the specifically spiritual benefits I can tell you that the Walking of the Gates will give you an experience similar in many ways to the planetary initiations that RO talks about undergoing with the 7 planetary Archangels, just with a rougher more atavistic twist.
The really neat thing about the walking rites is that it is one of the newest and most modern rituals for doing it, but uses some of the oldest dieties that mankind has ever interacted with (or invented or both). If you have ever worked Grants "Tunnels of Set", there is a certain element that is similar. The powers behind the gates are alien and mildly unbalanced, it is your work to balance them and move on. I have also used the 50 names to great effect in practical magic.
Those that hate it because it:
1. Claims to be the Necronomicon and an ancient text, which it clearly is not*.
2. Does not adhere strictly enough to Sumerian or Akkadian or Babylonian sources for the likes of whoever has their panties in a bunch about such things
are missing the point entirely.
*I should note that Simon maintains the story about it being in the Cashe of books. he has never said anything privately to me about this that contradicts his story.
I just realized that I only answered part of the question. The other one was about Gates of the Necronomicon. Gates is an excellent intro to how the Necro system works, and provides a good insight as to why it is important for the great bear to be hanging in the sky during the walking. The seven planets are each linked to one of the seven stars in the big dipper. This taps into a current of stellar magic that exists in Taoism, and of which there is some evidence of in some Mithraic temples as well.