OVER THE PAST few days, the Medicare.gov Web site has been jammed. Today, it could grind to a halt. That's because it's the first day of open enrollment for Medicare Part D, the Medicare drug benefit for seniors. Already, anecdotal and polling evidence shows that many seniors are baffled by the benefit, which requires beneficiaries to choose from several dozen private plans. More than six in 10 told a Kaiser Family Foundation pollster that they understand the new drug benefit "not too well" or "not at all." Only one in five said that they plan to enroll. Worse, more than three-quarters said they have never used the Internet, although the Medicare.gov Web site provides the most efficient means of comparing the different plans.

All of that bodes ill for a benefit that its creators have promoted as the wave of the future: the first example of 21st-century, public-private health cooperation. But as seniors start signing up -- and they have until May 15, when a late fee kicks in -- it's important not to judge the plan by the inevitable initial glitches and anticipatory fears. For one, it is possible that seniors, by gravitating toward the simplest plans, will force the system to become simpler.

In the long term, the benefit's success will be measured in at least two ways. The first is by cost: to seniors and to the government. At the moment, the average senior will pay about $32 in monthly premiums, some $5 less than predicted. The average government subsidy per person is running at $94 monthly, rather than $109. These savings are the result of better-than-expected competition among private insurance companies, many of whom got better-than-expected deals from drug manufacturers. The real test, though, is whether those are lasting savings or whether the insurers will find it necessary to raise costs over time.

The other (related) measure of success is how many seniors enroll. Former Louisiana senator John Breaux (D), one of the benefit's congressional authors and now chairman of the Medicare Rx Education Network, reckons that the benefit will be a success if half of eligible seniors enroll. But that's just an estimate: The real test is whether enough healthy people stay in the program to keep insurance companies interested in offering it and to keep premiums low.