Thanks to all for the great information.
I was looking for some advice on whether to repair or replace our water heater. Let me preface by saying any repairs will be "outsourced" to a plumber. I can probably drain the tank and change the temperature. That's about it.
Our current water heater is a Bradford-White 50 gal natural gas (M150L6N10). It was installed about 12 years ago (It is in the garage). During the summer (Portland, OR) it works well. There is plenty of hot water. However, the past couple of winters it has been very tempermental. On some mornings there is enough hot water (two showers), but on others we are racing to finish our showers before the water turns cold. I am not sure if there is any correlation to the outside temperature.
Any thoughts on whether it is worth fixing a 12 year old heater or might be wrong with it (we haven't done any maintenance on it over the years). I received one estimate of $860 to replace with another BW, but haven't had anyone come look at the old one.