Running time was longer after ice slurry (50.2 ± 8.5 min) versus cold water (40.7 ± 7.2 min) ingestion. Before running, rectal temperature dropped 0.66°C ± 0.14°C after ice slurry ingestion compared with 0.25°C ± 0.09°C (P = 0.001) with cold water and remained lower for the first 30 min of exercise. -Medicine & Science in Sports & Exercise
MY COMMENT: This is a significant difference in a run trial under an hour, and as senior investigator Paul Laursen mentions in this New York Times article, the effects won't last long enough to benefit you in a marathon.
Without some research, I am not so sure. Aren't you better off, if after an hour of running, you are a mile further down the road? And what happens if you take some Slushie not just before the run but during the run? I can see Nike Slushie machines on marathon courses.