Black and yellow un-metered shared taxis ply within city limits. These are faster and are always shared; rates may vary from INR 10 to INR 50 maximum. Being shared taxis, drivers often pick up additional passengers en route to the same destination, in order to make a better fare. If one decides to negotiate the "hire" of a taxi, however, the driver will not pick up any more passengers or stop roadside to find more but the fare will be significantly higher than if one had just hailed the taxi as it passed by.
For sightseeing it is a good idea to hire a taxi cab for full day. The fares should be fixed before travel. Hotel staff will be able to give you an idea of reasonable rate.
Much of the infrastructure is basic and many roads outside city are in bad shape due to heavy rains and poor maintenance, but within the city the roads are quite good. Although the city traffic ranks second in the country in terms to vehicles to population ratio, Mumbai being first, it is very slow and disciplined unlike most of India.
For places within city limits, city buses are good options. There are two kinds of buses that pass from Shillong - city bus and bazaar bus. City buses are what tourists should use. Bazaar buses are for villagers coming to Shillong to sell their products. Most of times they carry vegetables, fruit, poultry, etc, along with people. City buses are notoriously slow. Drivers move their buses from the bus-stop only when the bus gets packed or another bus arrives. Many times you will find that walking will take you earlier to your destination. Bus tickets are at most INR5 for one journey.