I flew on a couple US flights recently, crediting the miles to my UA account, and was pleasantly surprised at how well their system seemed to mesh with UA's. I called US before my flight to check that my UA number was entered correctly in their system (it wasn't -- but they very quickly and happily set it right) and to ask a couple other questions. While chatting with one of their people, I asked whether I would need to show my UA elite card to use priority checkin, etc., or whether they could see my UA status in their system -- they could. I used priority checkin and preboarding without incident, and actually without showing any UA credentials -- my US boarding pass had my status printed on it.

So, the much shorter version: no problems taking advantage of any of the usual perks. :-)

And incidentally, while US flyers are getting a lot out of the partnership, I actually don't feel like we're getting too terrible a deal, either. US's network on the east coast is nice, and I'll be using it a fair bit this year, crediting at least some of the miles to my UA account. Works for me.