A very good action film that is well crafted and has a good balance of action, romance and thrill with a highly interesting and layered plot, despite the overdone bad-boy/good-girl trope. It does everything very well, directing, writing and acting.
As a result, it has become a classic of HK cinema, with several iconic and memorable scenes, sometimes emulated in other films.

Story:
Nothing new, a bad boy falls for a good girl and it changes his life , trite but appealing. It has, action, romance, bromance, gangster power-struggles and a heist story-line as the cops-and-robbers trope. Yet, every part of plot line is done with depth and detail to captivate the viewer.

Writing, Directing, Acting:
What makes this film better than others and not just mundane is the good writing of a layered well told plot and the excellent execution by the director.
The characters are well-drawn, they come to life through the excellent acting of the cast, headed by icon and legend Andy Lau.

The ELEMENTS which make this film a a success and a “CLASSIC” :

--- SCENES ARE LAYERED ---. they are multi-purpose ... you think it is an action scene, but something else is also happening, another fact is being revealed, etc.... Every shot or scene gives relevant info and further unfolds the plot.
This excellent writing and directing is probably the most important factor in putting this film above the rest.

--- INCREDIBLE UNDULATING PACE .... the Tempo is like a dance ... Quick Quick Slow ... Quick Quick Slow .... most of the slower moments are the "Moments of Romance" ... . a perfect break from his violent life.
Just when you think you’ve figured out the scene, something interrupts ....

-.-- SEVERAL PLOTS SEAMLESSLY INTERWOVEN --.- 3 plots running simultaneously: the romance plot, the jewelry heist plot w/ detectives in pursuit, and the ill “godfather”, the triad boss, w/ a struggle for succession.. Of course, they all collide in the end!
Each plot-line given enough time to illustrate the essentials and captivate the viewer. Yet, a lot is said or expressed w/ "seemingly" very little effort .... I say “Seemingly”, b/c it is a Well Crafted Script!

--- WELL-DEVELOPED CHARACTERS .... we find out about their background, their hidden desires, their pain and dilemmas , and thus their motivation .... All these facts are incidentally revealed in scenes about some other action in the plot.
Again seamless interweaving of plot-lines and layered scenes.

--- LAYERS AND SYMBOLS --- especially the colors red and white everywhere ... symbols of innocence and passion, angel and devil, purity and evil ... all represented by Jojo and Wah Dee.

-— ICONIC IMAGERY —— Images that are immediately indelible and identified with it. A few examples: The final shot of her in the street. Andy’s bike outfit. And the head shot of the hero as he rides away on the bike , taken from below to make a powerful impact.
Some of these shots are emulated in other films, and where immediately recognized by me. What comes to mind, among others, is Taiwanese film “Our Times” , where the motorcycle scene is intended as a replica from this film, since FL was a great fan of Andy Lau and should recognize it and the effort of the ML without the need of any explanation or of any character mentioning the film or the actor,
Thus, to assume that even today’s audience would recognize such an image, further illustrates the lasting impression and iconic value of the imagery from this film.

--- PERFECT ENDING —- very fitting
--- PERFECT MUSIC —- fitting, memorable and recognizable.
--- PERFECT TITLE —- As expressed in the tempo of the pace, his respite in his fast-paced life is “A Moment of Romance” with Jojo whenever he can grab it , to make his life worth living.

NOTE: It took a second watch for me to appreciate the exquisite crafting of this film.