One of Wal-Mart Stores Inc.'s (WMT) most vocal critics actually has something nice to say about the world's largest retailer.

But Andy Stern, the president of the Service Employees International Union, is not about to get carried away.

"I think it's very good that Wal-Mart is starting to do some positive things," Stern said Monday in a meeting with reporters in New York.

Stern, whose union funds Wal-Mart Watch, a group vociferously opposed to the retailer's employment and business practices, cited Wal-Mart's announcement that it is re-examining its impact on the environment, including looking at the increased use of alternative fuels. He also noted that the company has had some success in diversifying its leadership.

Asked whether such comments weren't a bit out of character, Stern said his group has an obligation to acknowledge the truth of improvements in Wal-Mart's behavior. Then he quickly noted that Wal-Mart, in his estimation, has a long way to go.

"On one hand they have done some very good things. On the other hand they have a business model that's really not good for American workers," he said.

The latter is exemplified by Wal-Mart's announced plan to replace more of its full-time hourly workers with part-timers, most of whom will not stay long enough to obtain benefits, Stern said.

A spokeswoman for Wal-Mart said Monday the company welcomed any praise from Stern, but believe his union and others are making fundamental mistakes in judgment.

"I would love to be able to say it is good he is finally coming around and we appreciate the fact that he is able to see how Wal-Mart is good for working families," said the spokeswoman, Sarah Clark. "Hopefully, they'll stop attacking us and let working families decide where to shop and work."

Stern said the SEIU is working with other critics to formulate a specific "agenda of positive change" it will ask Wal-Mart to embrace. Up to now, critics have largely pushed for broad change, but have not set specific benchmarks, he said.

The critics plan to announce such specific targets in the next few months. Those standards will address matters like the degree to which part-time jobs are appropriate for the retailer, and also likely call for independent verification of changes Wal-Mart has announced to make sure they are more than just rhetoric, Stern said.