thanks all for your replies. i should have included more details - i leave in Guangzhou Zhujiang new town, and am traveling with my wife and our one-year-old daughter.
anyway, we just came back from fu tian. took the high-speed train from Guangzhou south station (40 minutes only to Shenzhen) and transferred to Shenzhen metro to fu tian. the queue to enter Hong Kong took us a good 30 minutes, although others all worked out pretty fast.
wasn't very happy with Guangzhou south station as it's far from city, therefore we took Shenzhen metro to lo wu on the return trip. the train from lo wu to Guangzhou east station took 80 minutes. had kfc upon arrival at Guangzhou east station before taking metro home. the whole journey took us 5 hours and 30 minutes. not bad.
Shenzhen North and Guangzhou South stations are "in" their respective cities, but quite far from typical center-city hotspots. At least they are both connected by their city's metro, but the transfers are unpleasant to say the least. Not to mention, with time to kill at Shenzhen North, I walked around the waiting hall, and naturally the smoking lounge was filled...so people just started smoking outside of it. 就这样
This may not help the OP in this case, but I used to frequent a lesser-known border crossing in Shenzhen called Wen Jin Du (Man Kam Tou)- kilometers-wise it's very close (to the east) of Luo Hu. The queues were short, but on the flip side, you'd have to wait for a bus (which went to Sheung Shui in Hong Kong). Additionally, there is a Wen Jin Du coach station, with buses to a few points in neighboring Guangdong cities (but perhaps not further than that).