They cost more than regular Medicare without delivering better health care overall. (The higher premiums are due to administrative expenses, profit, etc.)
That's why Dems are taking another look at them.
http://en.wikipedia.org/wiki/Medicare_(United_States)
With the passage of the Balanced Budget Act of 1997, Medicare beneficiaries were given the option to receive their Medicare benefits through private health insurance plans, instead of through the original Medicare plan (Parts A and B). These programs were known as "Medicare+Choice" or "Part C" plans. Pursuant to the Medicare Prescription Drug, Improvement, and Modernization Act of 2003, "Medicare+Choice" plans were made more attractive to Medicare beneficiaries by the addition of prescription drug coverage and became known as "Medicare Advantage" (MA) plans.
Traditional or "fee-for-service" Medicare has a standard benefit package that covers medically necessary care members can receive from nearly any hospital or doctor in the country. For people who choose to enroll in a Medicare Advantage health plan, Medicare pays the private health plan a capitated rate, or a set amount, every month for each member. Members typically also pay a monthly premium in addition to the Medicare Part B premium to cover items not covered by traditional Medicare (Parts A & B), such as prescription drugs, dental care, vision care and gym or health club memberships.<10> In exchange for these extra benefits, enrollees may be limited in the providers they can receive services from without paying extra. Typically, the plans have a "network" of providers that patients can use. Going outside that network may require permission or extra fees.
SNIP
Each year many individuals disenroll from MA plans. A recent study noted that about 20 percent of enrollees report that "their most important reason for leaving was due to problems getting care."<15> There is some evidence that disabled beneficiaries "are more likely to experience multiple problems in managed care."<16> Some studies have reported that the older, poorer, and sicker persons have been less satisfied with the care they have received in MA plans.<17> On the other hand, an analysis of the Agency for Healthcare Research and Quality data published by America’s Health Insurance Plans found that Medicare Advantage enrollees spent fewer days in the hospital than Fee-for-Service enrollees, were less likely to have "potentially avoidable" admissions, and had fewer re-admissions. These comparisons adjusted for age, sex and health status using the risk score used in the Medicare Advantage risk adjustment mechanism.<18><19>
In December 2009 the Kaiser Family Foundation published a report that rated Medicare Advantage organizations on a five star scale. The ratings were based on data from CMS, the Consumer Assessment of Healthcare Providers and Systems (CAHPS), Healthcare Effectiveness Data and Information Set (HEDIS) data, and the Health Outcomes Survey (HOS). New plans did not receive ratings, because data was not available. Almost six out of ten (59%) of MA plans did receive ratings, and these plans represented 85% of the enrollment for 2009. The average rating was 3.29 stars. Twenty-three percent of enrollees were in a plan with four or more stars; 20% were in a plan with fewer than three stars.<20>
Twenty percent of African-American and 32 percent of Hispanic Medicare Beneficiaries were enrolled in Medicare Advantage plans in 2006. Almost half (48%) of Medicare Advantage enrollees had incomes below $20,000, including 71% of minority enrollees.<21> Others have reported that minority enrollment is not particularly above average.<22> Another study has raised questions about the quality of care received by minorities in MA plans.<23>
The Government Accountability Office reported that in 2006, the plans earned profits of 6.6 percent, had overhead (sales, etc.) of 10.1 percent, and provided 83.3 percent of the revenue dollar in medical benefits. These administrative costs are far higher than traditional fee-for-service Medicare.