D4 is for driving like a maniac when you are too lazy to use SST. D5 is for driving like an old lady. SST is in case you are cool. Reverse SST is in case you are a downright awesome bad-ass.
+1 lol.
Actually, I don't like d5 in town. It seems the car tries to use the highest gear possible, and I hate the downshift lag. But I almost exclusively use SST. I like knowing what gear I'm in.