Was there in June and loved it. Stayed in Trastevere which is probably the best part of town to stay in - lively, full of excellent restaurants and used by Romans rather than tourists (you're going to find the reverse around Navona). Beer is about € 6 and a drinkable bottle of wine from perhaps €10.

Didn't bother with tour guides and was very happy with that - although others do like to have them. Personal preference really. If you don't go for guides then do get the €20 ticket as it saves messing around for the busses and trams. Also bus/tram/tube are the fastest and easiest and cheapest ways to get round, taxis are very expensive and a car is a total pain (parking!). The city is quite compact and no problem to walk around with the occasional bus for longer trips or if you get tired.

Don't rush it "ticking off" the sights, take your time and enjoy.