They are air-conditioned, which is vital in the summer, whether the bus is packed or not.
They are frequent and the routes often overlap for convenience.
They are crazy cheap, especially if you use the MRT before or after, since there’s a half-off deal that is the best thing to hit transportation since the wheel.
I haven’t met a hateful driver yet. Every morning, I take one of three possible buses to Chinese class. I always hope I get the 99 around 7:30 because the driver is so funny. He rocks a sexy late-night jazz DJ voice as he announces the stops (most buses have an automatic voice, but I get the feeling this guy disabled his so he could talk more!), and he says good morning and thank you to everyone that gets on and off the bus. He also occasionally poses questions to the riders, such as “Is our bus too cold?” I have to get his name next time I’m on that bus.
The only real downside I can think of is that I haven’t yet figured out the numbering system, but that’s not their fault.
And maybe wallpaper?