In the 17 years I've been a member of at least one FF program (starting with AAdvantage back in 1986), I have to say that in general my experiences most closely mirror Boraxo's assessment with a few differences noted below. I have redeemed or tried to redeem awards on AA, UA, CO, US, DL, AS, and NW.
Here is my view:
AA: 95% By far the best. They always seem to have something available where I want to go and when I want to go. Almost always exact date, although times may vary.
UA: 90% but declining. Again, very similar to AA but a tad more difficult. From the looks of things, award availability seems to have declined a fair bit in the last two years, but that may be due to everyone trying to get out the door at the same time due to UA's financial troubles.
US: 70% and may be improving. I've had decent luck in redeeming US awards, although I have not tried for any international flights.
DL: 70% Limited experience, domestic only, but OK success. Usually have to vary dates by a few days.
CO: 30% Absolute worst and no improvement in sight. International and Hawaii BF is nearly impossible no matter when you call. Even domestic coach is extremely difficult unless you want to pay ExtortionPass double miles. For these reasons I value my CO miles at less than 1 cent each. With the DL/NW/CO alliance, I will delight in using my 400K CO miles on the other two carriers.
AS and NW: I have very limited experienced with these two, but both have been positive. From the times I have checked availability at their websites, I would estimate them at around 80-85%. AS is probably a bit better than NW.