One thing that can happen with the SPX is that you can really get a surprise occasionally. Today, I nearly fell out of my chair. I was closing out some puts spreads on a rolldown and the midprice was $1.85. Needing to finish up the trade rather quickly since I needed to be back to work in a few minutes, I placed my closing buy at $2.00 (giving 0.15 to the vultures rather than my usual 0.10 ) figuring that it would go through in short order. It filled almost instantly--within seconds at the most, but when I checked the price, I actually was able to buy them at 1.80 -- 5 cents better than the midpoint!! This has never happened before, but I suspect that someone else at TOS was on the other side of the transaction selling at the same instant. With high liquidity and good volume, this obviously can happen.
On the whole though, I prefer the fills on NDX. What I would really like is strikes at 10 dollar intervals to improve the fine tuning of spreads. Any one out there who can make that happen?