It's still done, but manamune really isn't a viable pick on anybody anymore. It just takes too long for it to stack up. Situationally, it's still plausible for it to be a useful item, but rushing it is the only practical choice, but it requires too much time for it to reach full stacks. 186 times it needs to stack for it to reach max stacks. 2 stacks every 8 seconds. That means it takes 744 seconds approximately to go from no to full stacks, assuming a spell or aa is done every time it's possible. Or over 12 minutes. Assuming you can rush the 2100 gold necessary to build it at 10 minutes (you should get around 5000 gold if you never miss a creep, never get a kill, and no objectives are taken by 20 minutes, so 10 minutes is a generous guess for when you should get that amount of gold), it would take you at least until 22 minutes to get full stacks. A more relevant power spike can be earned with just sheen at around 8 minutes, or in almost a third of the time. And that upgrades into trinity by 20 minutes anyways, so the power spike happens again.
A.K.A. you might as well just get a trinity force.