Most G-gauge cars come with plastic wheels as a cost savings for the manufacturer.  The question arises, why convert to metal wheels?  I am in the process of converting all my rolling stock to metal and will list what I have discovered.
       In the testing of metal wheels vs. plastic, it has been proven that metal rolls better than plastic.  In other words, your rolling resistance is lower with metal wheels.  This will allow you to run longer trains on level track.  This lower rolling resistance is offset by higher weight, which means that fewer cars can be pulled up steeper grades (we can always double-head like the prototype).  Higher weight does improve racking, especially on lighter cars.  This weight from metal wheels is a plus when you consider that it will give you more weight at the lowest point possible on cars such as tank cars, flat cars, and empty hopper cars, which have a tendency to be too light and easily derailed.  How many times have you needed to add weight to these type of cars and could not find a place to hide the extra weight?  Of course you could put in loads, but I like to have some "empties" to deliver to my industries.
       One of the problems with plastic wheels, is that they leave plastic "crud" on the rails, which can cause some serious electrical contact and cleaning problems.  This is especially true outside where the rails get hot from the sun, and the wheels heat up naturally from friction.  If you are operating with battery or live steam, this is not a problem, but according to a Large Scale survey, over 90% of us use track power.
       Metal wheels are expensive, but if you buy in bulk, you might get a great volume discount.  Most modelers will buy metal wheels when they buy rolling stock and do the conversion before the car is placed on the tracks.
       Many metal wheels come in unrealistic colors (brass or nickel) and must be painted.  Real railroads do not paint their wheels.  The wheels rust and get covered with oil and grease through service.  Solid roof brown or grimy black make good wheel colors and can be easily air brushed on both sides of the wheels.  A rubber o-ring can be used to mask the tread and flange.  Do not paint the tread as the paint will foul your track.