2 things- flow and the gate being open.
The Piedra is a long day if you have to boat from above Pagosa, but there's no way around that until the Forest Service opens the lower gate. I have a friend that works that office and she might be able to tell me when the gate might open. Otherwise, you have to deal with 11 miles of casual boating with only one or two decent but benign rapids. Makes for a long day, unless you have high flows. I think it's still worth checking out, just from a scenery factor.
The lower section is fun but it doesn't pop until after 2000 cfs on the guage. After the gate is open, the logistics get a lot easier and it's a decent wilderness play run, in my opinion. If you're a studly V boater who doesn't want to bother with a IIIish run, then skip it. I think it's a classic because it's a free-flowing, beautiful river with a hot spring on the side of the river (at low flows). There are not quite enough big rapids at most flows, but it can get a little spicy in high water. I caught the peak flow in '96 above 3000 and that was a riot. We should see a peak around 2500 to 2900 this year (a guess, if it gets hot quickly) and if you live there you should definitely check it out.