on a day-to-day basis, I use a leather Coach messenger bag. Has a shoulder strap for the trains/subways I need to take.
I'f I'm meeting with a client and need to impress, I'll bring a Tanner Krolle briefcase. Definitely more traditional, but has nice details and the leather is great.
E90 TiAg 330i ZSP, ZPP, ZCW 6sd Manual, Nav & Satellite (for Stern!), Black leather w/ Poplar