After the rockfall hazard sign, the road is nearly covered with boulders, some small but some the size of a car! There was a path cleared out, just big enough for one car to drive around the rocks. In a few places I had to get out and move a few rocks that had fallen, but the path through the rocks was generally easy to follow (utilizing parts of both lanes and at times the shoulder). The last half mile had several large snow patches, but I'm sure those are gone by now.
Seeing several other cars parked next to the trailhead on Friday night was a comfort - I wasn't the only crazy one, and there would be help moving rocks if more fell!
Disclaimer: if you decide to drive to the Whitney Portal and a huge rockfall happens while you're hiking and your car is stuck until June, you didn't get the idea here...
In the morning, I started hiking up the trail with 4 other guys - 2 of them soon turned off to go towards Iceberg Lake and the mountaineers route, while the other 2 were going to do the standard route and then snowboard down!
The snow was solid from 8500' up, and I put on snowshoes. Although I have never been to Mt. Whitney before, the routefinding is very easy and there are few obstacles to going along the general path of the summer trail. I had planned to camp at 12,000' and then go to the top the next day, but it was only early afternoon when I got there so I kept going. The slope from 12,000' to the trail crest at 13,600' is quite steep and the snow up here was much more solid, making crampons useful. (This is the slope that is, looking up, to the right of the summer switchbacks). From the crest to the summit, the trail was clearly visible and largely snow-free except for a few large and annoyingly steep and mushy piles of snow. I slept near the top and then hiked down (mostly sliding) in the morning.
Many people have done the hike up to Whitney this winter - there were very few days without signatures in the register. New Year's must have had quite a party on the summit - dozens of people signed the register...
The spectacular winter view is well worth the effort.