2015-W Proof Gold Buffalo

MOST bullion coins, proofs, work BEST if you are buying in the direction of the trend of the metals.

That said, SOME bullion/proof gold/silver coins manage to retain their value OVER spot.

So I would say, IF this coin has managed to retain its original mint offering price and climbed above it, it is a good coin to buy.

IF however it has slipped below its initial mint price offering it probably isn't a good investment.

so again

1- its rare when a bullion coin can retain its value over spot

2- buying bullion/proof coins such as you mention to tend to rise with the spot price but beware they can fall hard with the bullion price.