I use the grid edit for tweaking midi notes and its just so good! I think my favourite thing is inserting control changes and being able to draw curves with the Q links.
I didn't know that feature existed at first, thought "oh I wish I could draw curves with Qlink" and then discovered that actually I could.
I used to a computer for sequencing and it would zap my creativity very quickly, but using the MPC is so useful and makes me more creative!
I don't use the grid edit for drums unless I want to lay down a pattern which my fingers can't do in realtime. (I'm not very good at the pads)