travelling around Tokyo late at night

Is it easy to travel in Tokyo after the metro has closed, eg if I go for a few drinks in Roppongi, will it be easy for me to get back to Shinjuku?

Yes, taxi service is available around the clock.

Yeah and the taxi will cost more than the rest of your trip - ha ha haaa :p

Seriously, I'd be checking the last time timetable and making sure I get on it. Otherwise a few hours sleep in the park and catch the first train in the morning.

Really, you don't wanna do this - unless you have money for taxis.

taxi's aren't cheap, but I don't think they'll break your bank either, unless you come over penniless to begin with. Japan isn't cheap. Drinks often cost around 700 Yen (more than 7$ US). I think the taxi from roppongi to shinjuku should be about $20. Money well spent compared to sleeping in a park. Assuming you have a hotel, I'd rather tack on another $20 to the expense vs. paying for a room and sleeping on the streets.

Instead of risking to pay for expensive taxis that you may have to queue for, most locals tend to either stay there until the first train, or go back by the last train and drink in the neighborhood. Normally, a healthy person can walk 2 or sometimes 3 stations worth.

Well because of the distance between Shinjuku and Roppongi is not very far, the taxi won't break the pocket book and run under 3000yen in my guestimation.

Alternately if you are up to it, you can walk back in about 60minutes or so.