Subways
As with the buses, the trains in Rome run on time ok, but nowhere near frequently enough.
The Rome subway network is much simpler and smaller than that in, say, London or New York. As a rough guide, it's basically a cross-shaped intersection of two train lines, one running north-west to south-east ('Linea A') , and the other north-east to south-west (the 'Linea B'). They intersect at 'Termini' station.
The metro runs from 5.30am to 11.30pm, though it's not as useful as you might think, since its two lines are more directed at ferrying commuters out to the suburbs than transporting tourists around the city centre. Nonetheless, there are a few useful city-centre stations: Termini is the hub of both lines, and there are stations at the Colosseum, Piazza Barberini and the Spanish Steps.
Public transport (buses, trams and the metro) in Rome is so cheap that you probably don't really need a weekly or monthly pass ticket if you're only here for two or three weeks. The 'metrebus' ticket costs only €1.00. It is valid for 75 minutes of travel almost anywhere in and around Rome and you can change buses as many times as you want during that time. You are also allowed one subway journey during that period, using the same ticket.

Taxis
Official cabs are white (sometimes yellow, the old colour) with a light on top and a large yellow 'Commune Di Roma' shield-shaped sticker on the doors. The fare is indicated on the meter but can be increased by supplements for night fares or luggage. In case of a dispute over the fare write down the number of the taxi, the pickup and drop-off location and the route taken . This should then be taken or sent to the Rome Tourist Board (Ente Provinciale per il Turismo di Roma, Via Parigi 11, 00185, Roma, tel. 488 1851).

Walking
Really, if you're not in any particular hurry, the best way to get around Rome (from a sight-seeing point of view) is on foot. A walk round Rome is obviously more tiring than riding the bus, but you'll see a lot more. You don't necessarily need a guide. The city is a fabulously beautiful place and meandering through its streets exploring for yourself can be one of life's most delightful pastimes. On foot though you need to be wary of pickpockets and beggars. Rome is extremely labyrinthine and there are some places you'd be better advised to stay clear of if you're walking, so it's a good idea to invest in a good streetmap.