There are a couple of things which you will not get if you don't
do NS.Whether such things are important is up to you.
1)Higher salary due to NS.
Most jobs in govt sector and some in the pvt sector will
give you a higher starting salary(increase of about $300-$500) if
you have completed your NS.
2)Tax Relief
Those who have completed their NS will be eligible for yearly
NSmen tax relief as stated here:
http://www.iras.gov.sg/irasHome/page04.aspx?id=3380
It is important to note that not serving NS will not adversely
affect your chances of getting a job.Like what others say,it might
have some effect on chances in getting a govt job but private
sector wise,most companies cannot be bothered.Of course,if you plan
on taking up uniformed jobs in the SAF/SPF/SCDF,then you should not
expect to be accepted at all.You might have to reveal your medical
condition in future to your company if they have a policy of
providing medical insurance schemes to their employees.A more
serious medical condition will usually result in the company having
to pay a higher insurance premium and this might cause them to
avoid hiring you.
Outside of the working environment,you should be prepared for
people asking you as to why you were graded PES F in the first
place especially if you don't have any visible physical defects(eg.
wheelchair bound) but you will get used to it in time and working
adults are usually more mature when approaching such topics,hence
you won't have to worry much about them making fun of you.