Use caution
Between this thread, my previous experience and talking to the National people on the 1800 number ahead of time, I flew to Denver last Wednesday night with my Emerald Executive card in hand feeling great I could just go walk out and pick up a nice SUV with AWD (huge snowstorm from Wed-Sat and my plan was to hit 3 different resorts). Got to the National Lot at about 10:30 PM. Nothing but cars, they said there were no SUVs at all, anything on the lot that was an SUV was already reserved. Couldn't talk my way into one. Luckily it was just me and 1 other person, and we found a single AWD Ford Fusion available. Without this, we would have had to chain up, or cancel and get a reservation somewhere else for who knows how much.
Short version: If you really need an SUV, pay the extra and reserve one. We were 1 car away from having to drive 600 miles in the Rockies in a 2wd car which would have been poopy.