There doesn't seem to be any shortage of braking action reports out there, but how much of it is usable? If presented with a μ number, an RCR, or just plain text "good, fair, poor" or something similar, how do you translate that to getting the airplane stopped?
Most of the answer relies in your aircraft flight manual, but it helps to understand what the rest of the world is talking about.
What follows are quotes from the relevant regulatory documents, listed below, as well as my comments in blue.
[Aeronautical Information Manual §4−3−8]
When considering pilot reports, consider the aircraft type. It may seem counter intuitive, but it may actually be easier to stop a larger, heavier aircraft than a smaller one. More weight on the wheels can make it easier for a Boeing 747 to stop than a Cessna 150.
[U.S. FAA Order JO 7110.10W §4-4-3 ¶11] USAF has established RCR procedures for determining the average deceleration readings of runways under conditions of water, slush, ice, or snow. The use of RCR code is dependent upon the pilot's having a stopping capability chart specifically applicable to his/her aircraft. USAF offices furnish RCR information at airports serving USAF and ANG aircraft.
Our Air Force flight manual charts had performance numbers with Runway Condition Readings (RCR). An RCR of 23 was considered dry, 9 was wet, and 4 was icy. The exact number tends to change a few points here and there, but those are close. If you are at an airport with military airplanes, they might just have an RCR for you. Can you enter your chart with an RCR? Probably not. But it is more information than you had before you asked.
[Aeronautical Information Manual §4−3−9]
The Aeronautical Information Manual seems to discount any connection between MU and good/fair/poor/nil qualifiers and you hardly hear the term "mu" in the United States. But the ICAO calculated coefficient seems suspiciously similar.
Figure: SNOWTAM Format, from ICAO Annex 15 Appendix 2.
A SNOWTAM is a "Special Series NOTAM"
[ICAO Annex 15] ¶5.2.3] Information concerning snow, slush, ice and standing water on aerodrome/heliport pavements shall, when reported by means of a SNOWTAM, contain the information in the SNOWTAM Format in Appendix 2.
[14 CFR 25 §25.105(c)(1)(ii)] The takeoff data must be based on . . . At the option of the applicant, grooved or porous friction course wet, hard-surfaced runways.
[14 CFR 25 §25.109(d)] Accelerate-stop distance. . . . At the option of the applicant, a higher wet runway braking coefficient of friction may be used for runway surfaces that have been grooved or treated with a porous friction course material.
[14 CFR 25 §25.1533(3)] Additionally, at the option of the applicant, wet runway takeoff distances may be established for runway surfaces that have been grooved or treated with a porous friction course, and may be approved for use on runways where such surfaces have been designed constructed, and maintained in a manner acceptable to the Administrator.
I've flown aircraft that allowed pilots to consider wet grooved runways to be essentially dry. The Gulfstream 450 does not. The performance section of that AFM does not mention runway grooves at all. The only thing in our books on the subject appears in G-450-OIS-02. On page 19 it says: "For landing operations on a wet, grooved runway, data in this OIS will be conservative." This leads me to believe Gulfstream has not factored in grooved runways and wants you to use it as a safety pad. In the absence of an explicit statement, I would consider a wet, grooved runway to be a wet runway. If somebody wants to argue otherwise I would say they would lose if they broke something or injured somebody.
14 CFR 25, Title 14: Aeronautics and Space, Airworthiness Standards: Transport Category Airplanes, Federal Aviation Administration, Department of Transportation
FAA Air Traffic Organization Policy, Flight Services, Order JO 7110.10W, March 7, 2013
Gulfstream G450 Operational Information Supplement, G450-OIS-02, Contaminated Runway Performance, Revision 1, August 3, 2011
ICAO Annex 15 - Aeronautical Information Services, International Standards and Recommended Practices, Annex 15 to the Convention on International Civil Aviation, July 2010