I would compare a hud to someone having a notebook in a live tourney. They really only show you information that you can write down. Just easier that it does it for you so you can concentrate on the game.
Do they work... yes.
Are they needed... no
One thought is that some of the stats you should take with a grain of salt unless you have a sample size that makes the stat mean something.
Example: vpip/voluntary put in pot
The higher the % the more likely they are a fish or calling station... all around bad player.
Someone with a vpip of 100% but they only played one hand just means they played in one hand.
The flip side: 100% vpip and 350
hands. Means they say every flop for the 350 hands you saw them play. Which is a bad player
The examples are extreme. i have not seen someone with a vpip at 100% with decent sample size but you get the picture.
Since any
tells you normally would get from a live game don't exist online the only info you have is the stats of the players.
Holdem Manager 2 is what I use. ACR is the only US
poker site that I am aware of where the hud is useful (the others are all anonymous sites hence the stats are jacked).