In NC, USA, where I live, between hurricanes and freezing rain, "mini-disaster" power outages happen every few years. As a result, generators are popular. At our home we have multiple backup systems for heat and/or electricity (propane, kerosene, fireplace insert, electric radiant heater). Our 4000-watt Generac generator has come in handy a couple times. Rather than wiring it into the house, I just run contractor-grade extension cords to select appliances.

To keep it in shape, run it every now and then and follow the recommended maintenance intervals for oil changes, filter changes, etc. Put fuel stabilizer (e.g. StaBil) in the gas to keep the carb from clogging. The generator won't lose its generating power from not running it, but perhaps the brushes could corrode or bond to the commutator (if yours uses brushes) and you store it a long time without running it in a corrosive/humid environment?

For planning, don't forget oil and air filters are consumables, as well as gas. (The oil-change interval on mine is only 50-hours.)

If you're shopping, some pluses to consider are:
[list]
[*]4-stroke engines generally last longer and are quieter than 2-stroke engines (there are exceptions)
[*]cast-iron cylinder sleeves are good for durability
[*]brushless generators are better than having brushes
[*]Sine-wave output is better than noisy triangular- or square-wave output
[*]low-oil-PRESSURE sensors are better than low-oil-LEVEL sensors, and any low-oil-shutoff sensor is better than nothing
[*]peak wattage of appliances is usually much higher than steady-state wattage
[*]if you get the smallest generator you can live with it will be easier to move, be quieter, use less fuel, and cost less