My first Phish show was 1989 and my last was 1995.
There is nothing, nothing about their music that can be considered "progressive rock". It's noodling which is, at it's core, risky. Sometimes it hits and other times it misses. When they were on the same page they sounded pretty darn good, but when they were bad...it was really bad. Once they released that album with the horse on it or whatever, and got a video on MTV their music took a huge hit, and their fan base took a huge jump upwards...so, obviously, did their ticket prices. One tour you could walk up to the window the night of the show and buy general admission/floor seats for $20. In the blink of an eye it was nothing but gigantic 3 day festivals costing hundreds of dollars per ticket.
They are a jam band, a somewhat talented jam band - but their fans are annoyingly arrogant. Things like "Trey is the greatest guitar player in the world", "These guys are THE most talented musicians EVER" are statements a Phish-head will use. Oh, also if you don't like them you must be "closed minded". They seem to think that because the songs have extended jams, some people don't "get it"...when in actuality their music is very, very approachable to anyone that's ever played an instrument.
They learned at the foot of the Grateful Dead who, while not as musically proficient, were a better band 10 times over.