One of the guys I race with has just bought a Yok BX, he was running my DF-03 for 2 weeks before he bought the BX and he was getting on really well with it. He had a quick go with the 501 and said that went even better, even though it was only its second run and it still wasn't set up very well. Last week he ran the BX for the first time and was rather disappointed with it, it wasn't quick enough (due to under gearing) and didn't handle as well as the 501. I'm sure he'll get it set up to handle well, but the 501 just seemed to go that much better straight away. The Yok seems a lot more "picky", in that you seem to have to get it set up right before it'll go well. We shall see.
-Steve