As with many himes, it really depends on where you are as a player. W.Diana is really strong for an established water main with a fast team. Her 4t Vigor buff provides perfect coverage for all your bursts on a 4t burst team, and comes back just in time for the next cycle. If you're not at that point, however, then her effectiveness does drop off quite a bit, since now she can't fully cover your bursts cycle. She's still plenty usable, just not nearly as amazing if you have to wait for her buff to come off cooldown.