I used this service today, because the sender (Betcoin.ag) included zero miner fee.  Transaction was of course stuck in limbo for 8 hours, and probably would have never confirmed.  At that point I noticed there was no fee attached.  It cost me ~$2 USD to accelerate transaction 0660a3c06d84da2bef2a524b25abe05457b0901f1ffd7b7b4d70f241688961cf of almost 4 BTC.  It was worth every penny to me.

2 confirms within 2 hours.