Both true b+w and slide are more expensive to buy and develop at my lab, and c41 is like 6 bucks after discounts, but I have a roll of B+W in my Pentax ME right now and plan on trying a few rolls of velvia 50 soon. It is more just seeing some stuff in color vs all black in white is a nice change.
I posted in the best photography thread a few color shots and I really enjoyed your crit last time so please help me out if you can.