Well, all of the Big 3 have done their E3 conference (Direct for Nintendo), and it seems the majority consesus is that Sony "won." However, there is a minority that feel MS won. The main reason, in their words? The ability to play 360 games. As of now, it only supports about 15 games, but MS promises to roll out updates to support more games. Even if it supported 90% of games, I think it's time for a reality check, this is nothing but a really nice additional feature. Hardly the "game changer" some were trying to proclaim it to be. And hardly a reason for them to be counted as E3's winner.
Now, if this feature had been available from the start, it could have swayed a few people, but it isn't going to do much this late into the gen. Of course, if we look at the PS3 and Wii U, it's very clear that B/C changes little about how a gen will play out, even if available at launch. The PS3 followed the highest selling console of all time, the PS2, and came with B/C, yet it struggled in the beginning. In fact, it wasn't till after they took out B/C and were able to drop the price that the PS3 sales really started improving. Seems B/C didn't really matter to those owners. And an even more recent example is the Wii U, which also followed another incredibly popular console, the Wii. And while the PS4 and XBO launched with no B/C, they quickly passed the Wii U's LTD sales in a matter of months.
The real reason there needs to be a reality check is that there are still people today, after there has been time to reflect, that act like the B/C was a true megaton announcement that may change the XBO's situation. I could see that being the case if B/C was invented just for the XBO and had never done before. But, that is not the case. Instead, B/C will continue to be almost a non-factor. In the end, it truly is just a nice feature to take advantage of every now and then to play some classics. And/or to write down to support your side in a console list war.