Pros:
Considerably less crowded than summer, huge waterfalls and high water in the river, best waterfall viewing time for the year, weather improving constantly to reach “very nice” most of the time in May, with usually perfect weather that entire month, good temperature for early season hiking.
Cons:
Early season will have limited hiking trails open above the level of the Valley Floor, unlikely to access directly from the Eastside (Highway 120/Tioga Pass closed until early summer), can be cloudy, raining or snowing, especially in March and April, some services are not open in early spring, including possibly cannot visit Glacier Point until closer to May 1. Mosquitos are very active by May.