I would agree with saildog, the price is really not that different and MacENC is basically GPSNavX plus the ENC support so you aren't losing features.
I recently got MacENC over GPSNavX after doing the math on chart costs. I haven't taken it sailing yet, but I have been playing around with it a lot (don't want to be learning how to use it in unfamiliar waters in fog or something) and I'm impressed. I'd be interested to hear more from people who have been sailing with it. The NOAA ENC map site is pretty well designed for getting just the charts you need and keeping them updated. I was pretty impressed with that too.
The nice thing about the free NOAA maps is that, well, they are free and as far as I can tell if you download the regular updates they are also the most recent/uptodate maps available. (Is this true????)