Does it work ? Is it worth the money. WoW latency is getting intolerable. Is this a good fix?4
The simple answer: It depends.
The complex answer: It really depends.
There are very few of these services that actually work as advertised. For one, if your latency isn’t because of the long haul, but instead is in your local connection, it’s not going to help, ever.
Some services manage to find better routes to popular locations, and so you might notice an improvement. But quite simply, the fact that you have to go through X routers and cover Y amount of distance will impact your latency, full stop, and no service can break that physical limitation.
As far as specifically for WoW, you want to be careful how the service presents itself. If it looks suspicious (common example is unrealistic location hopping), they will block your account for account sharing. On rare occasions you can get swept up in the gold selling bans if they were on the same IP, but that’s very uncommon, as they’re using much more than just IP. I lurk on the customer service forums a bit too much 
Their official stance on it is “unsupported”. Does not violate any rules, but if there is a consequence as a result (you get banned, constant blocks, etc), it’s not their problem. 
It’s a good fix for me. I’m in Australia and the ping to Final Fantasy XIV’s NA servers is unplayable for me (disconnects and >900ms latency) without using a VPN. I’m not playing WoW anymore: when I did I just put up with the lag. Many of my fellow Australians and neighbouring New Zealanders used WTFast (especially for raiding), and none of them, in the 9 years that I played, ever had any issues with their accounts being flagged as suspicious (EDIT: however see the comment below, by PeZzy, regarding WTFast).
I’m using Mudfish now and I’ve found it fast and reliable and incredibly cheap (I paid 9 USD about 4 months ago and still have 8 USD in credit, despite playing for hours daily). VPN’s aren’t prohibited in FFXIV, but I had one account check from Square Enix. Basically they noticed my purchasing on their store was without the VPN, so my IP was different to the one I played the game on) and I sent them my computer specs and fully disclosed my VPN use, explaining why I was using it to play but not to purchase, and they immediatlely unlocked my account again on the store.
Edit: to add to the comment above about IP hopping, with Mudfish you can select which (and how many) nodes you use, so you’re not showing up all over the globe. I now use two nodes that are quite close to my physical location but provide vastly superior performance compared to my own provider.
Just want to point out that WTFast is not a VPN, it’s a proxy which interacts only with the exe file. Using a proxy might set off alarm bells when your login app and the game don’t have the same user IP. If you plan to play in any geography restricted Asian games, you’d want to get a VPN like Mudfish.
Appreciate the correction, thanks for the clarification! I’ll edit my post.