Firstly, if you are unfamiliar with Heads-Up Displays (HUDs), they are informational online poker tools that allow users to instantly display information about poker adversaries and their hand-histories directly on the poker table on their monitors. Information ranging from statistical data from tracking and analysis software databases such as Holdem Manager and PokerTracker 3 to probability occurrence info generated by calculator tools like Holdem Indicator combine to give users a great wealth of data. Just like its use in modern aviation, Heads-Up Display gives users sharper insight into their opponents' actions, allowing them to make better and quicker in-game decisions. Online poker players using HUD efficiently definitely have an advantage over players not using it.
But is it online poker cheating?
I think not. Simply because the highly efficient informational tool is available to all poker players online and is not cost-prohibitive. And as far as its advantage is concerned, its use alone is not going to make anyone a winning online poker player. It is only one ingredient of the recipe to being a winning online poker player.