
I have watched the ORIGINAL short film version of 15 (about 25 minutes). Some people actually call it the "Ah Beng movie".

I have also watched the full length feature of 15 (about 94 minutes).
-- in Singapore, and
-- in San Francisco International Film Festival 2004.

I am not sure how much credit should go to the producer, Eric instead of the director, Royston .... I would prefer to suggest the latter ... why?

Mentioned earlier, this movie actually began as a short film and had garnered several awards (e.g., the Special Achievement Award at the Singapore International Film Festival 2002). Because of that, Eric urged Royston to make a full length feature.

Instead of rewriting the script with more details in between, he simply added two more stories, with two additional cast (because two of the original cast were missing/ or had been captured by the authority). 

In other words, the original shorter version was actually the first story in the full length feature PLUS the ending song on the MRT (i.e., the subway) ...

** It is interesting to note that I (and probably most of my Singaporean friends) like the first story better, and feel that the movie should have remained as a short film. The two additonal stories kind of make the movie slightly draggy and less compact.

** However, it seems like most foreigners prefer the second story and third story better. Why?

I believe most Singaporeans would prefer the first story or original short film because it has a good mix of humor and sadness that many can relate to. However, due to localization, most foreigners will fail to grasp the jokes ...
e.g., the distortion of the national anthem
e.g., the education system and society we are in
e.g., the landmarks the protagonists visit when they are scouting for a building to jump
e.g., the lingo they used (a lot is really lost in translation!!!)
e.g., the last song on the train (which is rather meaningful in the original short film, but I recall it not being very well translated).
etc

I believe most foreigners tend to like the second and third stories better because they can relate to
e.g., drugs
e.g., tattoos
e.g., antagonism between two rival groups (in this case, the gangs who spoke either chinese/dialects or english)
etc

Noteworthy, the movie has caused quite some stir and controversy because
-- the original sountrack depict actual secret societies' chants (their mottos, locations, etc)
-- a frontal scene
-- ** Also, the movie has sparked some debate because some mutilation scenes were real and NOT enacted. Basically, the director passed the camera to these kids and requested they filmed their daily life or he simply asked them to speak and live as they normally would (i.e., without scripting) ... consequently, some scenes were indeed disturbing ...

In summary,
... I think the original short film version is worth watching because it is more compact and more powerful 
... in order to better understand the humor, you may like to watch it with some Singaporeans ...


PS: The DVD is available.