I have never had a problem with any Shimano rear derailleur, and I think all my family bikes have some kind of Shimano derailleur. Derailleurs I have owned or used in include Tourney, Acera, STX, Alivio, Deore XT, 2300, and Tiagra. The higher end ones are lighter and shift better. Hence, I would rather have a Deore XT than a Tourney or Acera, but they all work, and are pretty reliable, even over years.