I visit Union every trip to Las Vegas as a bartender I befriended at Brand (in Monte Carlo) has been working at Union since they opened in Dec 2009. She's one of the best bartenders I've seen - ever - and a pretty good egg too!
I'm partisan to the appetizers at Union. I've also enjoyed a steak dinner there and it was cooked perfectly, loads of flavor. It's not a cheap place to dine - but this is Las Vegas, and unlike years ago, nothing much is cheap anymore other than a shrimp cocktail at one of the downtown hotels!
Check the bar out at Union - it's a great place for food, drinks and fun.