Speaking from experience. Immigration and Customs are actually two different jobscope. Immigrations is under ICA, Ministry of Home Affairs. Customs is under Singapore Customs, Ministry of Finance. Following the police force, ICA is the second largest agency in the Home Team.
The functions have been seperated in 2003 to enhance the border security by moving into Home Team. ICA work is more than what you see as choping passport. This is a wrong impression. That is just one area of work and definately is the place you will work when u get your first posting as an ICA Specialist. How can 1 organisation function just by chopping passport... Once you gain experience, there is also enforcement & investigation work, working on technology, processing paperwork, raiding, etc. You will also get to carry firearms when selected.
Being a SGT in ICA is different from army. SGT do not have a team of man to lead. The rank given to you indicate you "specialise" in your work. Dun forget, not all documents produced by people are genuine. You will be trained to detect forgery. Following the ICA specialist track, you can lead a team when u reach Checkpoint Inspector (Warrant rank of army) and above. The overall team lead is still headed by the Senior Officers. Regimentation is currently not the culture in ICA, perhaps in future as it is still in transition phase in the Home Team. Being so, the environment is more friendly and you can really make friends. So as long you do what you need to do, you will definately have a great time. If you are hardworking, you can further study private degree and convert to Senior officer. Of cse, you need a maintain a good performance record all along. There are lots of such examples already.
Some brief stuff abt your first posting at checkpoint.. You will perform clearance work at counters which you get to see different countries travel documents. Do not keep thinking abt woodlands, there is other checkpoints like Tuas and Sea checkpoints for first posting. You will follow the standard procedures. Anything you dun feel right will be referred to your superior for further followup. After your first posting, you will have the chance to be posted to Airport for future postings. Rotation of postings is within every few years.
Following the new wave of recuirtment, you can see that more and more pple is joining ICA to make sure officers are not overworked. In short, the workload of ICA officers at checkpoints are quite heavy as they handled high volume of passengers daily. But once you get used to it, you will move on.
As now is recession period, it is good to join as it is more stable and nice to give it a try. Just remember, opinions from pple who dun work inside are not going to help much. Just hope that above help u.