Let me add my praise for IB's generally fast executions, and terrific commissions. IMO, these two facts alone make the slightly clunky TWS and Option Trader interface worth putting up with. And IB's new Probability Lab looks very worthwhile.
Thanks Marco for your explanation of adding or removing liquidity and how that impact the comm. I now understand how it is I could receive a credit for a trade's commission, which has happened to me periodically.
I have been pleased with my average comm for RUT, consistently low (using SMART routing), ranging from 0.19c to a high of 0.79c, but mostly below 0.50c. Considering SPX comms average around $1.20, this makes RUT part of my portfolio mix, every week (and I like their Thursday expiries as well).
btw, when I inquired of IB about commission breaks, they told me that once you hit about a $1,000 a month in comm, you can get a better pricing structure (details on the site of course), so there is definitely a better price structure for very active traders. Hard to believe IB's comms could get better than their standard pricing tier, but apparently true.