is this cassette and chain ring combination good in terms of number of teeth?
I read that as you already have a bike with this gearing, you're asking if its okay, not whether it will fit and work.

You might want to recount the teeth on the big ring.  42 is not impossible, but 44 would be more common based on the other two.

Assuming this is your 11-23, 11-12-13-15-17-19-21-23, and that you're on a 700 wheel with 23mm tyres, then Saint Sheldon says this at  http://www.sheldonbrown.com/gears/

Little Middle Big chainrings

4.2   5.9   7.3 (metres of forward roll per complete pedal revolution, 11 tooth)

3.8   5.4   6.7

3.3   4.6   5.7

2.9   4.1   5.0

2.5   3.6   4.4

2.2   3.1   3.8

1.9   2.7   3.3

1.6   2.3   2.9  (same, but for the 23 tooth gear)

So its a road bike with a granny gear for steep hill climbs.  The 42 chainring limits your top speed a little, and the 11 tooth rear will have mechanical loss of efficiency compared to a 12 or 13 tooth.

There is nothing wrong with your combination.  Use and learn what you want, and where you struggle. Personally I have a 25 in the rear and a 22 on the front, and I still struggle with steep 20% grades.

Once its a bit more worn and you know what you want, then replace the chain and cassette with a range to suit your riding needs.