The service is free.
I made lots of tests, and it works great,
Great to know its free and handy tool(service)
Do you really need THAT fast of a transaction though?
I searched on their page...and found exactly why this accelerator is being provided as a 'service':
that includes a minimum 0.0001BTC/KB fee to ViaBTC
They're just looking for money. Save yourself the hassle and conduct your tx's naturally

I guess increasing transaction fees from your wallet should naturally fix this but what's the difference between re-broadcasting a TX and pushing a tx to using ViaBTC
Rebroadcasting requires paying a higher fee. Using this system allows you to just make it priority (if they mine a block) without increasing the fee. So it's a free alternative, but may or may not make an impact, depending on backlog/whether or not they mine the block.