I've spent one day there, three years ago. If you love Disneyland and you're heading to Hong Kong, then go. I didn't have a holiday specifically to go there, but I live in Australia and have made a few US trips, each with Disney theme park visits, and stopped off in Hong Kong on the way home once so I could see HKDL (by myself - I couldn't drag anyone else there).
Make sure you don't go there with high expectations. My reasons for going there were really to tick another Disney theme park off the list, and for a slightly different Disney experience. When I went, they had Space Mountain, which was the big attraction, but after that, they had Buzz Lightyear's Astro Blasters, Winnie the Pooh, It's a Small World, Autopia, Jungle Cruise, the Railroad and not a whole lot else in terms of rides. You can get through the park in one day easily.
Besides the park having a serious lack of attractions, probably the biggest thing lacking (which applies to different extents to all the Disney theme parks outside of the original) is that it's not Disneyland, and won't ever have the charm and atmosphere of the original.
On the bright side, the Hong Kong train system is quite straightforward and it's fairly simple to get to (though not the shortest trip), and the convenience of good, clean public transport was fantastic. The park itself is set out quite well, with very large walkways, and the cast members are enthusiastic.
It's worth a look if you love Disney - just temper your expectations and you'll enjoy the day!