I like Phish SONGS; therefore I enjoy first sets. I like Phish jams; thus I also enjoy second sets. I don't like every Phish song, so some first sets I will enjoy more than others (based on the songs they contain, perceived "energy" level, etc).

One of my favorite things about a Phish show is seeing how they will weave the tapestry/paint the picture/any other stupid art metaphor, of which the first sets are an integral part. That said, I can also understand why people don't particularly enjoy them.