This makes me feel really old, but I was at Diesel the first weekend that it opened. It was my roommate's birthday and a friend was a DJ there at the time. The joint was pretty empty until 11pm, our drinks were free, and we were new to the "club" scene. I was too punk rock to be comfortable at what I thought was just about the swankiest place ever. We were up at the VIP tables. It was awkward. (I never made it to veteran clubber status, FYI. Thank god. I also later learned that Diesel was not really that swanky.)
So I've been at Diesel in many different circumstances: early in the evening, so late that it's early morning, on the dance floor, in the bottle service section, when a DJ was spinning terrible music (house jams), when a DJ was spinning okay music (some rap, some top 40), and when a live band was playing.
That said...While I don't think that Diesel is a good music venue, there aren't a lot of good venues in Pittsburgh, so Diesel is better than nothing.
As a club, I'm not a big fan--except for the occasions when I had free bottle service. When I'm wearing my killer red dancing shoes, I want a club that plays all of the radio hits, but with a little humor behind it--mixing Ke$ha with the occasional 80s throwback or that Gin Blossoms song I never can remember the name of. Diesel takes itself too seriously. On that note, the wait staff also needs to realize that the reason we're paying $90 for a bottle of crappy vodka is for, well, the service. The only time that I've gotten decent service from the staff was when I was mistakenly thought to be with some Penguins players.