I use a diceware[0] passphrase for my Keepass database. I was inspired heavily by XKCD comic 936[1]. My only issue with password managers is that they are a single point of failure and are juicy targets for hackers, so I usually vet them and audit them thoroughly before I use them. I am one of those rare people that actually looks at the source code of password managers to look for flaws in the implementation (I sometimes spot flaws and duly report them to the maintainers).
One caveat to diceware I never liked is how it wears out the keyboard over time as you have to type the same passphrase each time to open the vault (You would be surprised how many times I need to do this each day). I sometimes have to lock my database to avoid evil maid attacks when in a hotel for example. Of course I go through about three keyboards a year because of this, but I don't mind the cost if it gives me a crispy fresh keyboard each time. And did I mention I don't own merely one encrypted database, but many depending on different contexts and different devices?