I've taken it from both -- from Marco Island when we lived in Naples, and from Fort Myers Beach after we moved to Sarasota. I love it. You don't have to worry about driving down the Keys (there isn't really anywhere to drive once you get to Key West anyway), and you can start drinking on board.
I'm curious as to what the bad reviews say. The only "negative" I can think of is that seating can be tricky depending on when you board. If you board in an early group, you get your pick of the ferry. On our way back from a girls' trip, we were having too much fun at Dante's Inferno next door to the port and boarded late, so we had to sit outside for the entire ride home. And I've seen some people looking a little queasy, but I get motion sickness and never had a problem.
Hope this helps!