I haven't burned a CD in ages, so I can't really tell you much about the CD burn quality of this drive, but in my opinion buying a PX-B940SA is not a good idea.
Basically the PX-B940SA is a Pioneer BDR-205 rebadge sold at a higher price than the original Pioneer drive and at the same time 'offering' much poorer firmware support than the original Pioneer drive. The PX-B940SA is stuck on firmware 1.08 for ages, while after a couple of successive firmware releases the BDR-205 has now reached firmware 1.12. All this time Plextor did not bother to release an updated firmware.
If you were to buy this drive, it is better to buy the real thing (BDR-205) if you can find one, since it is bound to be cheaper... the only reason I bought this drive was because at the time the BDR-205 wasn't yet released in the UK, and I really wanted one.
Regarding CD burn quality, have a look at the various PX-B940SA / BDR-205 reviews and that should give you an idea of what they're capable of...