As for Alsa/another sound server replacing OSS, OSS do the mixing (and resampling?) in the kernel space, citing latency is one of the reasons, while alsa let userspace programs the jobs. IMO, that kind of works does not belong to kernel space, so I prefer alsa.
Regarding to pulseaudio, dmix is fine, but pulseaudio is better with features like glitch free playback (ironically, this is the reason why pulseaudio glitches so bad on some systems with broken drivers), you can set the resampling algo, per stream volume control, flat volume (another problematic feature), and as some people said, it is the only setup that allow output via bluetooth devices but I haven't tried it yet. The main reason for many problems related to it is the horrible audio drivers on Linux (as always), so you can't exactly blame pulseaudio, at least it always has fallback mode, and the distros never set them as default.
Back when pulseaudio was first integrated into Ubuntu (around 8.04, right?), it didn't work well for me and stop working for many other. But now, most people I know have absolutely no problem with pulseaudio.
PS: Aside from dmix, there are several other sound servers like arts, esd etc.... too, I'm glad that we get rid of all that and now pulseaudio on alsa is the standard.