The 3520/40s are superior readers, fast burners, and offer QS. Unfortunately, media stability at 16x isn't stellar, & i've already voiced my opinion regarding their reliability at speeds down to 8x.
For my money, I use a dedicated DVD-ROM reader (Aopen) for all my reading & QS chores. I did try QS on the 3540, but it kept returning very different scans of the same disc - unlike the Aopen. Perhaps this will improve with future firmware revisions.
My biggest problem with these drives is that at present I just can't count on them. Sure, <b>most</b> of the burns (i'd guess 80%) come off o.k., but the remaining 20% result in poor burns to coasters. The only coaster my 2510s ever gave me resulted from junk media of such inferior quality that their physical defects were visible to the naked eye.