I think the airport is something like 20 km from the city centre.
Immigration is very quick. You'll complete a landing card onboard the plane and as long as you have filled it in correctly immigration is a breeze, I think I was through within five minutes.
They basically swipe your passport, swipe the landing card, stamp your passport, and that's it...
As you haven't been to Singapore I can tell you it's one of the most efficient countries in the world.
Security is neither better nor worse than any other airports. Probably depends on the time of the day and the number of flights leaving.
It's up to you to decide whether to do it or not but I really do wonder what you intend to see during that ultra-quick visit to Singapore city centre.
From a taxi the only thing you will see is a lot of skyscrapers which look no different from any other skyscrapers in any other cities.
I personally really like Singapore but I think it's a city to experience more than to see, you will not get a fair impression of the city seeing it from a taxi during 30 minutes.
Especially the cultural diversity is amazing, there are as many as four official languages for this tiny little country.
Wouldn't it be possible to change the ticket and include a short stopover instead?
One day will be enough for you to see at least the most important attractions and get an idea what the city is all about.