The steptronic box is a wonderful box. when driven in manual mode it allows you to rev alot higher than you would in D mode. but it will not allow you to over or rev or downshift with too high revs or speed.
you can comfortably drive in manual mode as long as you like cos the shift motion you make is merely prompting the box to possibly shift. you are not actually shifting the gear, merely suggesting a gear change. if the car likes your suggestion it will allow it, if not, the car wont allow the change.
I currently drive a steptronic e90, and have owned two e46 steptronics previously, as well as an e46 SMG and an e90 manual.
Whilst the Manual does give you the most engaging drive, I still prefer the smooth comfort of the Steptronic.