Viabtc is a well-known mining pool that captures roughly 4 percent of the Bitcoin network’s hash rate.
I wouldn't use them personally, I don't like what they are doing using the free accelerator they provide, they are basically trying to make people support bitcoin unlimited and using their exchange could lead into the same thing and since you don't have control over private keys in exchange, you could be supporting BU without even knowing at some point.
Well, they are promoting what they believe is the right thing to do with bitcoin scaling issue. At least they are helping those who have stucked confirmation even thought we need to compete against other people who have unconfirmed transaction.
I think it is still good having a new cryptocurrency exchange. So far, their service are good and reliable, so I think it is a plus in bitcoin community for such reliable company to have a cryptocurrency exchange service.