Download the free 3-day trial from the official website. Play your main game for an hour with it on and off. If you see a consistent 20ms+ drop, then buy a 1-year activation key from a trusted source (like the official store or a high-rated eBay/Kinguin seller with buyer protection).
WTFast often allows users to try the premium service for a limited time (usually 7 to 30 days) before charging. This is the best way to determine if the service actually lowers your ping on your specific games before you spend money. wtfast activation key