CS5 is the most important release for Adobe in years: quantum leap improvements in performance, stability and features.
Premiere Pro and After Effects have been re-worked into native 64-bit applications, and can now really take advantage of 64-bit memory access, hugely important for HD and higher resolution workflows.
If that wasn't enough, GPU-accelerated playback (NVidia's MPE or Mercury Playback Engine), rendering and encoding are available with NVidia GPUs, increasing encoding and rendering speeds multi-fold, and allowing instantaneous full-resolution previews of complex layered timelines that were previously unattainable.