Having never played the original despite owning it, I found that getting the Definitive Edition with its enhancements and all the DLC to be a rather good investment. it certainly worked out cheaper than buying all the DLC on their own.
My first impression video goes through the graphics performance, improvements, and gameplay.
Given that I have been thoroughly enjoying the game, and the graphical improvement are extremely good. Especially the new density improvements making mini Hong Kong come alive and bustle with crowds.
Performance is good, although sadly so far AMD Crossfire doesn't seem to be working for me. Even so even with a single card, the game can easily be maxed out at 3440x1440 with AA set to Normal aka FXAA. I have not seen any improvement using the high AA settings at my resolution.
Word of warning though, if you use an Ultrawide monitor or multimonitor setup, all cinematics are rendered in 16:9. Resulting in pillar boxing.
None of which detracts from the game, and it is very good. The story is compelling, and there is a variety of sub missions, and favours which can be done. There is a small RPG element, allowing you to visually customise your character via unlocking or buying new clothing. Along with upgrading skills. Which are in three main categories. Police, Triad, and Martial arts.
They all complement each other, and levels are gain in each by doing their respective faction's missions.
If you already own the original and enjoyed it, but you're missing the DLC and want those. The Upgrade cost to the new version is easily worthwhile, and you gain the new improvements with it.
For new players I would happily recommend this as a sandbox action game, with a very good story, and great gameplay.