If Bitcoin can increase hundreds of dollars in value in just one day, how can companies and small businesses accept such currency?
Simple. A third party payment gateway converts it to fiat currency and automatically deposits it to the company's bank account. Actually it's very convenient for merchants to do it and it also has lower fees to the merchant than most other payment systems.
The payment gateway BitPay (which handles Steam and many other merchants) allows a merchant to accept the fiat value of the BTC they receive with only a 1% fee. By comparison, PayPal charges 2.9% + $0.25 to the merchant when the transaction involves goods and services.
Alternatively, the merchant can just receive BTC and hold it to hedge against fiat inflation. Overstock say that they keep 50% of the BTC they receive.
Nobody knows the surefire answer, but do you guys think now is a good time to invest?
A good time to invest was mid 2015. Hindsight is 20/20.
You could use cost averaging to buy when you accumulate fiat if you want to.