My company started offering this a couple of years ago, and I had the same issue you did - not a lot of information out there to help you weigh pros/cons. My company offered a $5K 'jump-start' contribution for each employee, and pays $1000 per year into the rmsa on behalf of each employee (regardless of whether you contribute or not). For the first $3K the employee elects to contribute, company pays an additional match of 50%. Contributions are after-tax but compound tax free and can be withdrawn tax free in retirement. Fund offerings are limited but good quality (Vanguard index funds). Company contributions are notional (expensed when incurred), and we can use the funds to pay for health care costs if we retire from the company at age 55 or later (and can stay in the company's group plan, but you have to fund the costs). When we become eligible for medicare, funds can also be used to pay for medicare premiums. If you leave the company prior to age 55, you lose the company's contribution (but your contribution stay in a trust for your use after age 65). Prior to retirement, you can tap your contributions to pay for COBRA premiums, but that is all. Another item to note is that if you die, the company's contributions and yours as well go back to the company - you cannot will it to an heir.
I ultimately opted to fund the rmsa to the point of the company match. I plan to retire at ~ 55 (in about 15 years), so as long as I stay with the same employer (no plans to leave currently), I should benefit from this plan. Even I do seperate from my employer, I can still use my funds after age 65 for medicare premiums. I would be careful about putting too much into the plan, given its limitations, and I would want to understand what funds were available first. If they are good, then this is another avenue to 'save' in a tax-sheltered way for retirement.