I have been to Mamak several times, and this is one of my firm favourites in Sydney CBD.
Mamak serves up a range of incredibly tasty Malaysian dishes, most notably a wide range of interesting rotis and curries. The drinks, though a little more generic in nature, are still quite tasty (the ais te and ais kofi are great). All dishes offer fairly good value for money
The restaurant has a warm ambience, although it does tend to get a little noisy. Not quite the place for a romantic date, but perfect if you're hanging out with friends.
The service is very quick, although a little rushed because of the crowds that inevitably throng the restaurant. A 15-20 min queue on a weekend is quite normal. But do invest that time, the wait is well worth it!