personally, sport mode is the best. steptronic shifts too slowly. sport mode (in my car) is when you just pull the shifter towards you (when its in drive). steptronic kicks in when you nudge it up (upshift) and down (downshift)

I personally think it not a 'good' substitute to manual or smg. But i do find it a lot better then the rest of the quazi-manual (Audi, benz, nissan,etc)....i find bmw shifting to be a lot more responsive and the torque is just sweet. If you do not like the manual 'pain' during commutes, settling with the steptronic is worthwhile.....all in all, i think its still sweet....cuz the engine sounds just as same as the manual when you switch gears during low end gear shiftin.....The best moment to use it is right off the line. Make sure you do not rev the RPM to the red-line, it will not be good for your engine. Shift it when you feel like the engine has maxed (before the red line)....but during a race on the expressway....shift the gear down when you see someone passing you....and it will be on 4 gear...then rev it up and then let go to 5....oh God, i'm getting excited talkn about this...back to work...bye.

If you are talking about the "pseudo-manual" mode that is activated when you move the knob to the left, I use it whenever I want more low end torque. You can shift it yourself by moving it forward and backward, but if you don't shift it will shift for you way before you redline.