So I used expedia.com.hk to rent a car in mainland China. Note that this is only do-able if you have a mainland Chinese license (search other posts about obtaining this in HK).
Arrived in Xi'an international airport at around 5pm, took me about 30 minutes before finding the place. The arrivals hall is in Terminal 3 but the car rental parking lot is in Terminal 1. The booking has clear info stating this, but it was a bit of a walk from T3 to T1. When I returned the car, the staff used the car to drive me over to T3 for departures. I suspect there's shuttles from T3 to T1 but I didn't bother and just walked.
Took the scenic route to my business destination. Took me 4 hours instead of 2 via mountain passes. Quite beautiful and there was no traffic (but suspect summer vacation months to be gridlocked with tour buses) and the roads were great without a single pot hole.
Be sure to download Baidu maps, it beats Google Maps hands down in finding places. Though I didn't look into finding their English version though (they must) as I downloaded the Chinese one.
On the way back chatting with the rental car company staff, he says there are tons of HK people doing self drive vacation now including "foreigners" who managed to get their hands on a mainland driver's license. The whole process was painless with no cash used in the transactions only my HSBC visa card. Paying for gasoline though locally you should bring some cash.
Stayed at a local hotel on the outskirts of town, can't complain.
Can't wait to try it farther up north when it snows and bring my wife and kids to show them the massive countryside that can be explored. It's a mindset one can't get if you grow up entirely in a city unto itself Hong Kong.