There are several good articles about which twitter users to unfollow or never follow, with very valid reasons. However, I have one particular type of users that I honestly dislike. That is fully automated twitter systems, combining several types of undesired behavior. It is very funny when people advertise automation and forget, that twitter is interaction primary and not single way street to puch content.
The worst kind of offenders is easily to spot on twitter: Users, whose all posts are links to some content they push from specific feeds. The most annoying kind is pushed by twiterfeed, which pushes quite a lot messages at once. This effectively spams my timeline with crap I have read couple times before. Usually, there is not even a single reply, retweet or linkless message in their whole timeline. Their business is elsewhere – maybe in their blog/website, maybe they do not know what to do with their twitter account. I follow some of them for single reason only: if I am moderately interested in specific market, they can work as news feed in single place for information about niche.
Another quite funny type of bots are people who use periodical reminders for their specific content/products. They are relatively harmless if they provide some useful content between their bot messages. I doubt that the person who read the same message 5th time is more likely to buy product advertised than the one who read the message 4th time only.
Automatic systems do not work that well without some brains. You can advertise about tools that allow one get 20 000 of followers, but everyone will laugh at your message if you have only 1000 of them. You can push automatic content into twitter, but it will not be read as long as it is not relevant or fresh to your readers.
Neverless, using Hummingbird or similar tool is ok as long as you look before auto-following people. You have to decide what are they interested in : you or just follower count.