The Re-Groove agent is brilliant, u just select your track on the sequencer, click a few buttons on it is routed through - there are about 32 groove channels.  There are also a number of preset grooves which are suprisingly good and have a profound effect on the sound - u can either keep it real time or commit the groove to the sequencer - only tried it briefly on a basic rhythm section and I was more than impressed!

 

The sequencer has been completely re-built - this is a bit of a frustration because it doesn't work in the same way - the new autmation procedure is intuitive but again it is very diff than reason 3 - I know it is better but I supposed I need to settle into it. 

 

Summary:  For the new synth alone, I absolutely love it.  The groover is fantastic and is very easily accesible.  The sequencer will take a bit of getting used to again, but it looked the bomb.    Apart from that (and more refills across the board, some nice drum-kits)  it is essentially the same as Reason 3 so don't expect a completely new package.