It is very accurate, although about one minute delayed. I live on the approach to the easterlies at LHR. Zooming in, I can locate my house, and watch the little yellow planes fly over. The little yellow planes pass over about a minute after the real thing.
Have you ever heard the phrase "live and let live"? After 45 years of aviating, I'm still an enthusiast, and love watching flightradar24. And yes, I do have a life!
Although the arrivals for 09L fly over my house, the noise from 95% of them is not a problem. When the other 5% come over, a quick look at flightradar24 always shows that they are failing to achieve a CDA.
I live in between the Ockham and Biggin holds for Heathrow and have a similar thing on the iPhone (Plane Finder).
When the ash cloud thing was all happening it was extremely accurate. As the flights were starting to arrive back at heathrow on that first night when Willie Walsh told the CAA to go take a flying @%#! and the following day, when I heard a plane overhead, I could see it right over my location on the Plane Finder map and tell where it had come from and what the passenger in seat 21J had for their meal.
It only seems to work over Europe, is there an equivalent one for the Middle East, it would be handy to spot where the AC are while waiting to pick up passengers. The local flight arrivals is dreadful.