I am reasonably familiar with them but doubt that they are anywhere near mainstream. Please note, I am still learning about them and don't claim to know a great deal about them even though I have bought them before the run up to $1,200 and after the fall to the levels below $650. More businesses in my area and across the country are accepting BTC in particular. I'm a little surprised that digital currencies are not more popular but there are a variety of efforts underway to make them so.
They can be useful particularly because there is little or even no cost in sending money in this form and there are lots of sites you can shop on using them. I've even bought Christmas gifts and gift cards using BTC. I think that if more people who send money to family or friends want to avoid bloated fees looked into this option, they would see some benefits over other options. Those touting that digital currencies are primarily used for illegal deeds and products are wrong. Mostly law abiding people use them for legal products and services. Further, those who want to do away with digital currencies only take this stance because they are a threat to their moneymakers somehow such as credit card companies, banks, WesternUnion and Moneygram. Those who have no money to lose may fear something they haven't taken a bit of time to learn about to make a more informed decision for themselves.
The prices have been holding steady for several months after the run up to $1,200 and the drop to below $650. They have been holding steady at around $575 - $640 per BTC for several months now. I would guess this is the sweetspot for BTC but I really don't know. I have held other digital currencies but always come back to BTC. I even ask for them as tips on my HP profile :)