Recently we were holding at 270 and the CI hold speed was 191. Would you have held at 191 or at 250? It is just that 191 seems awfully slow.
The holding speeds equate to basically the minimum drag speeds, and are safe. They are essentially the same as the driftdown speeds. (LDmax for those old flight instructors.)
The data behind the cost index speeds is sound, as long as you remember to put your zero fuel weight in the perf init page on the FMS.
The speeds are safe, especially since the cost index numbers are around 15 on the 200, and 24 on the 700. That means that the speeds it spits out are well above the min drag speeds, and way above the 1.3 Vs speeds.
If you follow the cost indexing to the letter, it is possible that you will burn easily within 100 lbs of planned burn. (That number may be off by as much as 500 due to the downwind on arrival, or the departure turn if leaving ATL.)
Starting today, all flights will have 500 contingency and/or a minimum reserve fuel of 1900 until any fuel/flight planning problems are worked out.
The important thing to remember about cost index is that it is saving OVERALL cost, including crew costs, etc. Those are included in the calculation, and as fuel drops in price, it is less of a consideration. The calculations start with cruise flight, and go back from there, so whatever climb speed you need to get to your cruise altitude at your planned mach, it will give you.