Vel'koz is kind of fun, but the hit box on his Q seems wonky to me. And I swear it seems to sometimes explode out on nothing at all (without being activated or anything). I've done decently well with him, but I've also destroyed him in every match I've been against him. One thing I only really noticed after playing against him is that his mana costs seem really low, and he can sit in lane and spam abilities for a very long time with little issue.
His real strength is in his comboing, since W is instant cast. W>E>Q and you've chunked 1/4 to 1/2 their health easily. But after playing as him, then playing against him - I felt like his moves are a bit too predictable - especially with the Q indicator. I could just side step a tiny bit to dodge all his abilities then start fucking him up easily.