Over the past few days, I listened to several of the new extra channels. Here are my thoughts:
Classic Rock BBQ was pretty good. I'm not sure why it would be different than a "party" playlist, but that's what they call it.
I listened to a 60s-70s-80s pop-rock channel, because they don't otherwise offer a channel that covers that span. It was okay, but it got bogged down in 60s songs I wasn't getting into (House of the Rising Sun etc).
They have an 80s pop-rock channel, which (again) is not exactly offered on existing channels. I bailed when it overloaded on hair bands, Def Leppard, and Judas Priest.
The 90s pop-rock channel was really good, but it was definitely "poppier" than a pure rock channel. However (and this becomes a theme), they run out of songs and start repeating.
I listened to a 1st Wave top tracks channel. Generally good music, but it starts repeating after a while. If you look at the extra channels, most are either "top 100" or "new discovery" tracks. So by definition, it's a smaller playlist.
The oddest one was the XMU Discovery, labeled as new emerging indie rock. OK, so I normally listen to XMU everyday, and they mix-in recent and classic indie music with the latest music available, and I mean latest. I figured that XMU Discovery would feature the newest songs from the regular XMU channel. But out of 10 songs, 7 were songs I never heard, from artists I never heard of. While that may seem better, it's not. That's what I didn't like about Pandora - the music they played had no relation to what was getting buzz, or up-and-coming artists. It was just a random smattering of songs that had a certain "sound" to them.
Overall, I think many of these channels would be good for parties, workouts, and other limited time-frame events; but not for long-term listening.