While waiting for an answer, I'll chime in with my very limited experience on ICs that I've traded on other indices. I've traded on the following:
SOX
MID
MSH
RUT
While Dr. ZhiVodka is no doubt right on the difficulty of getting filled at the mid on the SPX whereas the others may be much easier, with every other index I've been forced to adjust. All the other indices were relatively more volatile and always approached my short strikes more frequently than the SPX. This caused me to have to consider adjustments and often adjust. I have not lost any money with the SOX, strangely enough, as you would expect this one to be volatile. I'm ahead on the MSH, but have lost some. The RUT was a disaster, as the MMs are obnoxiously stingy and tightfisted and won't give an inch. Strangely, the MID was also a disaster, which surprised me as I expected it be somewhat similar to the SPX but it was much more volatile than I expected.
These are indices so volatile is a relative term (as opposed to stocks).
So maybe I'm not getting the best fill on the SPX but I think it gives me the best chance to avoid adjustments and keep the entire IC credit. If you have to adjust or get out because of a move against a short, you almost always lose some or all of your profit if not a lot more. So if I don't get a nickel or a dime on the SPX because it is not competitive, it still seems to me that it is worth it, because my odds of having to adjust or get out all together are lower with the SPX than with the other indices. So, to me, it's not just a question of best premium fill (of course I want to get as much as possible), but also safety of the index chosen. You can view it as a tradeoff, but I make good money and sleep better at night with the SPX than with the others.
This is just my experience, but I do know of another trading group that pretty much has had the same experience.
Quote from dsandraz:
Could you give few examples?