For me, 60% of my client from Upwork agreed to pay in Bitcoin.
However, it's against Upwork's rule to be paid outside their service.
You should be extremely careful about this, especially when you chat with your client
They will review your chat log and lock your account.
When I used to work with Odesk (now Upwork), I have one clients who pay me in directly to may Paypal accounts. I know its against their TOS but we have make it a point that we will not used their tools to track our chats.
But I've been away for so long that I don't know they there are clients now that are willing to pay their workers with bitcoins. Yes, you should be very careful if the clients decides to pay you in bitcoin. But I'm really surprise though.
I've been a Odesk worker since 2009 when it was still young and only few workers. I was lucky though that I have maintained a client for 3 long years. Sadly though, it has to end so after that I never entertained any clients and my odesk account has been sleeping for more than 5 years now. I would say that I made tons of money back then.