Some choice quotes:
"25 MySQL Boxes to 10 Sphinx"
"50M queries per day w/steady growth"
"1,000+ qps during peak w/room to grow"
It's great to see it standing-up well, though it looks like they made (or sought) patches for issues they ran into.
One of the first changes we made to sphinx was just to increase the minimum word length so that queries could be extremely long (as error messages sometimes are quite long). This was changing a define and tweaking some other areas internally.
We also made some changes recommended by others in the sphinx forums and have been adjusting the weighting algorithms for our own needs.
For adjusting the maximum query length, you can find tips on that in the sphinx forums.