I've been using a 3.3 hp Mercury for 5 years now. The first year it drove me nuts. First, the shear pins were way too wimpy. If I hit any sort of vegetation I was forced to change the pin. I finally resorted to a steel pin. Then, I had some trouble getting it to run consistently. Toying around with the choke, I found a 'sweet spot' that runs really well.
Since then, I've had zero problems with it and over the last week have convinced a couple of buddies that having one is a really good idea. We were pushing in a mile or so into the marsh and poling/paddling with strong winds can be a real pain. One thing I do is I always shut off the fuel flow and run the gas out at the end of each trip. A buddy recommended doing this so that there is no fuel in the carb and lines which can (theoretically) gum things up.
Scott