The current research
tells us roughly 1/6th of all active players in the world use tracking software, which usually contains a HUD. The HUD is useful for productivity and helps you multitable, but the real value is in the tracking software which allows you to improve as a player while reviewing your sessions for errors post-game.
Most players who multi-table use them - otherwise it is too hard to keep notes on your opponents.
precisely. We always say the HUD is optional, but you would be foolish to not use a personal tracker - its the fastest way to improve your game, but at a minimum you should use one to allow you to track your winnings or losses.