I'm glad to see posts like this.
A lot about witnesses and their role on the Steem platform was already written, but due to changes (Steem is evolving) some of information might be outdated. This is a good opportunity to refresh that subject.

Please note that witnesses are not only responsible for Steem's heartbeats but also for it's evolution (I.e. choosing proper ways by accepting or rejecting changes to rules proposed in so called HardForks)
Vote for those that in your opinion are representing your best interest.
Feel free to ask your delegates about their opinion on various issues you are wondering about. First hand info is always better than gossips and misinformation.
Most of witnesses can be found on steemit.chat's #witness channel
I will be back there too, as soon as I get back from vacation. I like @joseph's explanation.