First, let's distinguish between HUD and tracker and the value of each.
Do you multitable at all? Do you play enough MTTs at one site often enough to run up against a lot of the same players regularly? Do you avoid taking notes on players because it's just too much of a hassle or too awkward in an online environment or just too distracting to your thought process?
If your answer to any of those questions is yes, then you can certainly benefit from a HUD. Even if you answer no to all of them, you may still benefit but it probably won't be worth as much to you.
Trackers, on the other hand, are
invaluable, especially to the beginning or intermediate player, whether you use their included HUD or not. Trackers allow you to study your results outside the game, identifying leaks and opponent tendencies over many
hands. They can help zero in on where you're spewing money, perhaps maybe you're playing too many hands from the blinds, or you're going to showdown too often. If you apply the effort to learning how to use a tracker to analyze your own play, your game and winrate will improve a drastically greater rate than just from using the HUD. But since the HUD is included with the major trackers, why NOT use it?
People get hung up on HUDs and how much they *think* they'll help you. They won't help you at all if you don't know how to read and apply them. Despite what a lot of the ignorant, rabid anti-tracker/HUD types claim, a HUD does not make you a winning/better player just by virtue of turning it on. In fact many beginners find it distracting and detrimental to their game because they're trying to figure it out while they play, or they become blind slaves to the stats. It can be a great tool but only when properly applied.
As mentioned, the cost of a tracker/HUD is so small for a serious player as to be a no-brainer investment IMO. It's 4 BI's @ 25NL, 2 @ 50NL. Even if you play lower than that, if you intend to take your game to the next level quickly through disciplined self-study, a tracker will get you there quicker than anything and will pay for itself in no time flat. Perhaps even a single session or two.