I think Ethereum is definitely one of the altcoins worth investing in. In the past, I did not find it right to invest in any coin other than Bitcoin for the long term, but when I saw the thousands of developers working on Ethereum and some of the needs of the industry, I understood that it is definitely a valuable asset. Also, I think the important thing here will be the distribution ratio of your portfolio. If you think you have enough Bitcoin, it wouldn't be a bad idea to buy some Ethereum. 
Having diversified investment is always the best choice. Ethereum stands strong as the best compared to many other altcoins available in the market for various reasons. The projects that hav got developed over the ethereum platform itself enough to determine the potential of ethereum. Maybe the growth is slow, but we'll have assured market progress in the long term. When it comes to altcoins investment one should know when the enter and exit, then it'll be the best with low risk.