At least in the US the NEXRAD radar is a major improvement over what we had for years. After using it for a few years it is obvious it is fairly accurate. Of course when an aircraft that is recieving data linked weather, requests to deviate they are looking at the same information the en-route controller sees. The data only updates every 6 or 10 minutes depending on the mode it is operating in.
There are still alot of coverage gaps, check out the coverage link on BendixKing's website.
https://www.bendixking.com/wingman/static/FIS/faq.jsp
For more information on NEXRAD
http://weather.noaa.gov/radar/radinfo/radinfo.html