I've been here several times for drinks and once for afternoon tea (which isn't *technically* in Mandarin Bar, but in the adjacent lobby area). It's the only place in Las Vegas where I've partaken of a tea service, but I couldn't recommend the experience more heartily...probably one of the most relaxing, pleasant afternoons I've had. Your fellow tea-goers are relatively hushed, the view is epic, the service is attentive and friendly without being overbearing (i.e. Mandarin-esque) and they serve legitimate clotted cream instead of butter. Big points for that.
The bar itself is probably best experienced (as others here have noted) after dusk, but you may be out of luck if you haven't reserved a spot ahead of time. I like to show up just after they've opened in the late afternoon and snag one of the primo tables that are never available after it gets dark. It may be slightly less magical without the twinkling nighttime lights of the Strip, but the view is still enthralling and the drinks are still fantastic.
My personal favorites are the house Golden Leaf and the Robin's Good, but in all honesty I'm pretty sure this is the only bar in Las Vegas I've visited where I've *never* had a subpar libation. Every time I visit I consider taking part in one of their Scotch or Japanese whisky flights...one of these days I'll take the leap.
Not literally, of course. It's a long way down.