You can access the Star lounges in international economy ONLY if you get your Gold Preferred card.

US has never sent me a card since the merger -- the last "Gold Preferred" card I've got is from 2005 (no joke!). I called US constantly looking for a card and they'd tell me "you don't need one, just show the boarding pass to the club people."

That's not true -- the Lufthansa club matrons, in particular, will NEVER let you in the club without an actual US Preferred Card showing Star Gold status.

Since US can never be bothered to get me a Gold card, I wouldn't waste the cash. In the best case scenario, you'll waste 6 months or more waiting for them to get around to giving you your card so that you can enjoy Star Alliance privileges that everyone else gets within a couple of days of hitting gold on other carriers.