Actually, A0 is the earliest stepping - which in most cases never goes out to customers because it can be unfused and I doubt these would see the light of the day outside the vendor's facilities. First samples to reach customers are A1 steppings (ES0) in many cases, but sometimes it starts with B0. Earliest samples are tested in a very limited sense and can have lots of issues but, as evidenced by the amounts of those available illegally on Ebay, it is pretty clear that Intel became damn good in their fabrication process since these chips are obviously good enough to be sold on Ebay by those ES-chip sellers.
The differences between B0, B1/B2 and B3 are in the number of resolved issues. Naturally, B0 has the longest list of issues since it is very early silicon. As the steppings increase, so does the number of tests performed on them.
The reason why IvyTown B-samples seem to be better in terms of compatibility is probably due to the fact that IvyTown is a "tick" generation, so many problems were resolved in the previous "tock" (Jaketown or SandyBridge EP).
@poulk, yes, if there is no C0, then B3 is the next best choice since it is the most mature of B-steppings. You can even buy 4 of them if you want and put them in a 4-socket motherboard for 48-core goodness
But, since this is an ES chip usual warnings still apply: it might be buggy for what you are trying to do and, of course, to get them on Ebay somebody in the process obviously had to break the NDA with Intel, which means these chips are sold illegally - so do not count on any support from Intel or motherboard vendor.Edited by psyq3212 - 11/8/13 at 4:55am