I'm currently with Xtra, on Go Large, (yes, yes, I know, but I'm in a 1-year lock-in), but can change provider in the next few weeks.
The time when broadband performance matters most is when I'm trying to connect, and stay connected (!), to XBox Live and Sony PlayStation Network services. Also, when in online games on these networks. My experience with Xtra is that XBox Live performs quite well, but online gaming performance is very average. PlayStation Network performance is, however, just truly dire, and practically unusable. I'm constantly getting logged out of PSN, disconnected from online games etc. Even using the online Sony Store and downloading content is unreliable. This is just totally unacceptable.
So, I'm in the happy position of choosing a new broadband provider. XNet is currently top of the list based on various forum feedback and their plan structure. However, I'll consider any ISP that's been shown to give good performance for these apps.