Hey Paul,
I have the navionics + card as well for those same areas you talked about, I fish lake st. clair a lot. It has a lot more detail then the old gold card had, but the detail isn't as good as say, Lake Erie. Depending on your unit, you also have to activate the "fish n' chip" layer to see the additional contours. The good news is they are always being updated and you can update the card as much as you want within the first year.
The card was worth the money in my opinion. I think it was $199 but if you have one of the older navionics chips already, you can buy the Navionics Update card instead (this is what I did), and its worth $110. Works the same as the Navionics + card, but they charge you less because you already have a chip.