Definitely. The low cost makes it a much more attractive proposition than the tourist boats too.I would highly recommend a boat ride from the Thames Southbank if you dodecide to go to Greenwich - the greenwich observatory, maritime museum, and surrounding views are really worth it (they also have a planetarium at the observatory which has some great short movies - the "blackholes" showing rocks!). Plus, from Embankment station, which is directly on the water, a boat ride from there would only be 45 mins max to Greenwich. The Thames Clipper is the fastest way to get there, and costs about £4 each way. During your ride, you can have uninterupted views of all the major landmarks - big ben and the houses of parliament, St Paul's cathedral, canary wharf and the "gherkin" building, etc... It's quite a refreshing ride on a hot summers day.
Just to add to this excellent travel advice. If you feel brave enough to catch a bus between locations then do so rather than catching the tube. The tube is convenient but you can miss a lot that you would see on the top deck of double decker.
If you want to catch the tube, catch it from Holborn to Tottenham Court Road. It's only one stop but you pass through the old, disused British Museum Station which is in darkness and if the train goes slowly is as spooky as hell.
Make sure you get a travel card (£7.50 per day) for all zones. It gives you the freedom of all public transport in London and lasts until 5:00am the following day so you can catch the night bus back while drunk (not if you value your life). Even better get an Oyster card. http://www.tfl.gov.uk is the best place for information on travelling in London.