New in Cubase SE3:
New VST 2.3 audio engine
Fully automatic delay compensation for plug-ins
Enhanced user interface (same “look and feel” as Cubase SX3)
Platform-specific performance optimizations
Improved Hitpoint detection for tempo adjustments of audio loops
Drag & Drop of MIDI tracks directly into the Project Window
Extended Track Inspector with improved track parameter access
Many editing and workflow enhancements
SE3 just came out, so I don't know of anyone using it yet. I imagine it will be a slight improvement over the current SE, if just for the upgraded audio engine.