I haven't heard any AM IBOC yet. Have tuned in the three or four FM stations transmitting in the area. Fidelity was not drastically improved, maybe a little better/cleaner high end. The most noticeable things were the much improved SNR (gone is the HF hiss common on FM analog stereo signals) and better stereo separation.
Did you tune in the stations on a conventional receiver, or on an IBOC one?
The (analogue dial) FM tuner in my house stereo amplifier has a mono/stereo button you can push to kill the stereo feature. If the station is not absolutely full quieting, there is much less hiss in the mono mode.
I have noticed the NPR station about 50 miles from here has a constant gurgling sound in the background when the receiver is in the stereo mode. It is very distracting and sounds like a weak unstable heterodyne in the background, very similar to the sound of an AM carrier with the SBE effect. It's not the receiver because I hear it on another receiver with digital tuning (not IBOC) as well. In the mono mode, the background sound disappears.
I don't hear the gurgling carrier on the car radio, and it receives in the stereo mode all the time.
Back to the topic, here are a couple of interesting websites regarding IBOC:
http://www.analogzone.com/nete0819.htmhttp://www.rwonline.com/reference-room/commentary/2006.07.19-03_rw_hd_guest_barry_2.shtml