Hello! 

Please tell me the noob what is the difference between IsValid and != null. I've noticed that IsValid doesn't works correctly in some cases when I add check for objects/npcs/players, for example like this: var ganker = ObjectManager.GetObjectWoWPlayer().FirstOrDefault(g => g.IsAttackable && g.GetDistance < 150 && !g.IsDead);

Thanks in advance for answers!

IsValid checks if it still exists in objectmanager. You never need it, unless you store a unit over a long period of time.
Because when you move 100 yards from the point of unit acquire, it might have disappeared from the objectmanager and not been updated either.