I usually only use it when there is a real problem with ISP routing such as when undersea cable having issue. During such time the routing is very congested in the normal route but WTFast can route your connection via other country which is lightly used and hence improve your ping. During normal usage, WTFast normally can find slightly better route for you, but I don't find it significant enough to really care about it.

You won't get banned for using it because it merely changes how your network traffic reaches the server.