How good are those odds?
If by "good" you mean the chance is 249,999/250,000 that it won't hit us, that's pretty good.
Meaning should we be concerned?
It's nothing that should disrupt your day to day life.
Could it cause mass extinction to mankind? Can it destroy the earth?
No. It would certainly cause some regional devastation, but such an impact would not be a mass extinction event.
Check here. This calculation uses the estimated size of Apophis (270 m) and some roughly standard assumptions about conditions in an asteroid impact scenario. It places the observer at a distance of 100 km from the impact site. You can find the damage projection near the bottom.
The major seismic shaking will arrive approximately 20 seconds after impact.
Richter Scale Magnitude: 6.7
Mercalli Scale Intensity at a distance of 100 km:
VI. Felt by all, many frightened. Some heavy furniture moved; a few instances of fallen plaster. Damage slight.
VII. Damage negligible in buildings of good design and construction; slight to moderate in well-built ordinary structures; considerable damage in poorly built or badly designed structures; some chimneys broken.
…
Glass windows will shatter.
As you can see, no mass extinction and no significant damage beyond a few dozen kilometers from the impact site. If you'd like to experiment with you own values, go here.
Is it to large to be deflected?
Any attempts to develop a method of deflecting or rerouting an object like Apophis will take this into account. It isn't too large to have its orbit influenced, and certainly isn't too large to blast. How to do those things is a problem to solve.
Thanks this has worried me for some time and caused a lot of anxiety.
There's really nothing to worry about regarding Apophis.
