My best experience, this one time when I went to check in for a Y seat and was told I'd been bumped up to J
(and it then kept happening for a while after that)...
My best experience when I've actually traveled in Y (excluding Y in PE seating on QF8) it was a AKL - MEL flight on a B743... The cabin was nearly empty so everyone had a row of 3 or 4 seats to themselves.
I put up the armrests on the other two seats and turned the seat into a "bed"...
My worst experience, there are a few that jump to mind, like the one from PER-SYD where the guy next to me kept on wanting to cuddle up to me as he slept. For the record, I am a straight male who was in his mid 20's at the time, the guy doing the cuddling was prob in his late 40's and not overly attractive looking. To make matters worse, the guy in front reclined his seat whilst we where still boarding and I was placing my laptop under the seat in front. Nearly got knocked out by the seat coming down on top of me.
Another pretty bad experience was a AKL-SYD, it was in a B767, and the seat was pretty much stuffed, and I got sat next to the pax from hell. I finally moved to a center row seat after the seat I was seated at finally gave up the ghost, and was much happier. (unusual for me, normally I hate the middle row of seats on a 767)
The third pretty bad experience was an AA flight from MIA-LAX, it was a B757 (it was the flight which has given me a hatred of the B757), to call it a sardine can would give it a sense of space and freedom. I was squished in, next to another big bloke, with a cabin crew who didn't want to be there, and reminded you that they didn't want to be there at every given opportunity. It was a little bit of a shock to the system after flying J SYD-SFO a week earlier (I don't really remember the SFO-LAX-MIA legs as I pretty much passed out after a night on the town previous)