Investing in bitcoin is a risk, as the value can crash tomorrow.
I'm holding too so far, I believe in what bitcoin represents, so I'm taking the risk. Don't invest more than what you can afford to lose however, but if you can, you should buy more.
Also have a look in other cryptocurrencies.
Investing Bitcoin today is quite risky because sudden drop of price would cause panic. But in my observation the price today caused by a natural phenominum that it realy soaring high because investors is realy flooding to invest and hold. For the year 2018 I think it would continue to rise
and its safer to invest from the start of the year.