If you do not use a HUD, you are just at a disadvantage. I don't know why you'd want to consider playing seriously giving yourself a handicap when the games are tougher than ever.
As the creators of the first commercially viable HUD (Poker Ace HUD) and of course PokerTracker - it may shock you to learn that we do not agree with this. The HUD was designed to replicate a live playing experience, in a live game all human being have a built in poker tracker in us, we remember our opponents and how they play. In an online game all we have to remember our opponents is a screen name, human beings are not "programed" to remember screen names, the more opponents you come across the harder it is to remember who your opponents are. You could replicate the HUD in your brain by taking a tremendous amount of notes and playing in a very small field of opponents, for example not all nosebleed players use a HUD - they don't need it because they know their opponents and play the same people frequently. But for the rest of us, that is just too hard to do - hence we automated this by creating stats and placing them on the table so you can use this information to create what we can an "opponent profile". Each stat
tells you very little by itself, for example you don't really know what cards your opponent are playing when they have a 19% VPIP (see below for an explanation); but this information combined with the data from other stats can help you piece together the puzzle of who your opponent player is, and how they may be playing.
A 19% VPIP means the opponent is playing 19% of all hands, however this does not mean that opponent is playing the top 19% of all hands as shown in PokerStove. PokerStove uses a ranking system called Hand Vs Two Randoms, this is a hand value weighting system to attempt to mimic how a real player may value their hands, it was created at a time when the primary game played was limit holdem. In PokerTracker 4 we allow our users to create their own hand weighting systems, to help mimic real world player styles - for example a player may balance the top 2% of their value range by adding the bottom 2% of their air range to remain unpredictable, or they might 3Bet with hands like A3s to provide balance in their range - this cannot be represented by a single stat percentage, this can only be determined through observation and understanding of how to create that opponent profile in your own head while playing - the HUD helps you do this, but the truth is if you took a tremendous amount of notes and only played at one table, you may find that your results are the same or very similar - proving that the HUD is an incredible tool, but you are not at a disadvantage if you chose to not use one provided that you are playing at a single table, your a great player and you take great notes. With that said, anyone who is multitabling would probably be better off using a HUD.