Did a few basic tests with 2 Plextor drives and some Taiyo Yudens I purchased from SuperMediaStore. Thought I'd try to briefly summarize with some very high level results regarding only visible burn rings and readability in the 2 Plextor drives:

Drives:
* Plextor PX-708A
* Plextor PX-716A (via an External Enclosure connected via USB 2.0)

Media Used:
* Taiyo Yuden DVD-R 4X (Code TYG01) - Purchased from SuperMediaStore and came in shrink wrapped Value Package

Results:
Burned the exaxt same source with the Latest version of Nero and found the following...

PX-708A
Burn Speed 2X: Burn appeared to be flawless in looking at the burn side (no visible rings) and is recognized easily by the PX708A when reading it.

Burn Speed 4X: Noticeable burn rings on the underneath side and the PX708A drive had extreme difficulty reading the disc after it was burned. Still plays fine on a DVD player though.


PX-716A
Burn Speed 4X and 8X: Both speeds here looked very good at the bottom of the disc and both Plextor drives quickly recognized the discs after the burn.


Summary:
With the PX-708A and using the Taiyo Yuden DVD-R 4X burning at 4X, the burn completed successfully although the drive that burned it could hardly read it and I noticed "burn rings" underneath. I have never experienced this with my Ritek DVD-R 4X discs. Hard to really tell how much of a problem this is though because all of my DVD players play it fine.

Not too much detailed info here however I may run some error checks with the Plextools software when I get the chance...