Yes I replaced my O2 sensor not long after I bought the car, about 10Kmiles ago. Yes I can see it working. The will jump around at times, but when cruising it nice and even, I have a slight miss very now and then and you can see it in the A/F guage.
For tuning it is very nice to have. I going to have either a EGT sensor or a vacuum gauge next, but I'm not sure yet. The vacuum gauge is much cheaper, but less useful then the EGT.
But I had hoped to have the car painted by now(been work on it since spring) but it is get cold here, so I way just put off any type of mod intill the painting is done.