Are you asking about an HSA or about a HDHP? Lots of people seem to confuse the two.
An HSA (Health Savings Account) is essentially an IRA account that can also be used to pay for medical expenses tax-free at any time. It's a great thing, and should be part of your retirement planning. I recommend funding it in full after funding your 401k up to your company match threshold.
A HDHP, or High Deductible Health Plan, is required in order to qualify for the tax-advantaged HSA account. It provides lower premiums by empowering you to take more control over your health care expenses. In other words, you'll pay less money up front, but you'll have to pay a larger portion of your expenses throughout the year. If you have fewer expenses, you come out ahead. If you have lots of expenses, you generally come out ahead, too. If your expenses are closer to your deductible, but not much over, the you tend to spend more on a HDHP. Generally speaking, HDHP's will save people money, but they're less predictable. It's kind of like the dining plan debates we have here on the DIS: traditional health insurance (and the dining plan) will typically cost you more money, but they provide assurance that most of your bills are paid. The HDHP (and paying out of pocket for meals) makes things less predictable, but generally saves you money.