This is a reasonably easy trail about 8 miles in length.  You can start from Happy Isle, and hike to Glacier Point, but unless you specifically want to walk uphill 80% of the time, some of which is very steep, then I would start from Glacier Point.  As you approach the last 3 miles of the trail, you'll reach a junction with the John Muir Trail.  Unless you are running late, I would turn right and go down via Nevada Fall.  The John Muir Trail between Happy Isle and the junction of the Panorama Trail is very, very steep and 3 miles long.  Going down will pound your knees and feet into powder.

It's very important to start as early in the day as you can, due to the propensity of stopping and drinking in the view.  You will probably have an extended lunch break, and generally slog along stopping and taking pictures, etc. 

The other great thing about this trail is that it is surprisingly void of crowds.  I was on it Memorial Day 2005 and the people on this trail were noticeably absent.

View of both Vernal Fall (bottom) and Nevada Fall from the beginning of the trail just below Washburn Point. A side view of Half Dome is in the upper left.

Illilouette Fall

Another great view along the Panorama Trail.

Notice the hiker in clogs...  Actually, she was wearing clogs only through the wet areas of the trail.  Still not a good idea.

Nevada Fall and Liberty Cap.

This portion of the John Muir Trail sends a shower of runoff onto hikers.