I saw this password strength post on XKCD, which was posted on 10th August 2011.
The summary at the end of the comic strip sums things up very concisely:
Debate rages across the internet in a ceaseless battle of opinions, concerning how secure this method of generating passwords is, but I wanted to have a play with the method anyway. I think the advice in XKCD is still valid, from what I have read online. However, doubtless lots of people could find evidence to the contrary.
There's a lengthy piece about the XKCD approach via explainxkcd.com.
If you want a more personalised passphrase, try the "yourword" passphrase generator. For example - type your own word, e.g. "andy", and submit the form, the sets of words which are returned in the passphrase each start with each letter in turn from the word "andy"...
29,134- that's quite a lot of potential combinations
Here's the XKCD comic strip post this is based on: