I have a 740s with sounder that I installed. It's older than what you have and I think they added faster software with the upgrade. Mine is fantastic. I would definitely get the sounder plugged in. In our waters in NC, there are alot of shoals and shallow spots. The depth finder has a nice adjustable log of depths and temps.
The GPS is decent, but in not all cases are the graphics for land in North Carolina correct. This is mostly due to our shifting barrier islands, so more a local issue than with Garmin. I am considering the bluechart add on, but really can't justify the $200-$250 for what essentially is a gadget upgrade. Could paint the decks and or bilges with that money...