I use a R25 red filter on almost all of my lenses when I'm shooting on the street with my Nikons. I've been shooting Neopan 400 and HP5+ at EI1600 with the red filter, getting roughly the same exposures as I would when I shoot the same films unfiltered at 400. The contrast and grain that are inherent of pushing the film two stops, along with the reduced brightness in the blue wavelengths gives me a -lot- of grit. I love this combination, but it is certainly not for everyone, and most people think I'm crazy for doing it, but when I print the negatives, I love the look.

Then again, I also love the way reversal film cross-processed as a color-neg looks when printed on black&white paper.

N.B. I use the aforementioned Nikon setup in conjunction with an unfiltered Hasselblad and normally rated films. I like having the ability to paint with 2 brushes at once.