Yes, I believe a 5 day trip to WDW is worth it. Last year, I took my fiancee' to WDW for 5 days. (This was his first time, I'd been there before.) We were running around for our entire stay, (by choice!), so he could see almost everything. Five days was enough time to see all four parks, and spend time at downtown Disney. (We did miss Fantasmic though.)
After we got back from that trip, my fiancee' mentioned to me that for his first trip, 5 days was perfect, but he did think that with so much Disney has to offer, that 6 or 7 days would be nice for future trips.
Diznee25