I think Mandarin works better as a mid-late game card than an early game accelerator. It's a terminal Gold, give or take. Not a bad card to play if you can spare the action/card. The problem is its on-gain effect.
In the early game, if you see $6, you probably want a Gold. If you see $5, your treasure quality is probably very poor (5 Coppers, 3 Coppers + 1 Silver, etc). Top-decking those treasures makes you wait an extra turn to cycle, which is a big penalty in the first two shuffles. Most of the time, it's probably not worth it.
In the mid-late game, the on-gain effect can work to your advantage, because (a) you probably don't care about buying another Gold with $6 and (b) you're much more likely to see $5, $6, or $7 (or higher in Colony games) with high-quality treasure. Play 2 Golds, buying a Mandarin and top-decking 2 Golds, hoping to line them up with Province- or Colony-buying treasure next turn. The down-side here is that you're missing out on a Duchy, so it's a bit situational. Like Courtyard and Haven, its effect is also quite good at smoothing out hands in the late game (though not as good as Courtyard and Haven).
I still haven't quite figured out the card, but my sense is that it's usually a trap in the early game and situationally nice in the mid-late game.
If you have sources of extra +buy available, I imagine the on-gain could be quite nice with Vineyards as well. Buy Mandarin + Vineyards, top-decking your Potion for Vineyards next turn.