You can certainly get great 8x burns on the 1633s, and 4x as well with the right media. But at the same time, 12x and 16x can be iffy depending on media - see other thread on Good Media for 16xx drives - and is limited to a few choices. Definitely 1st gen of 16x burners, and not as reliable as can be. Also, if you look at the 1633s/1653s reviews with Plextor scans, you'll also see that the laser burns aren't as well seperated as they could be -- they blur together in the scans (the scans where you have several 'hills' next to each other - the more distinct they are, the better the burns) - here, the Plextor even does a better job.
What this means is that while the burns may be error free, they can be hard to read on other drives and later in the future, esp. vs. other burners.
These points alone have already gotten me to move over back again to the Pioneers (this time, A09 up from A06, which I also have) -- here, the solid DL burns with very low PI/PO error rates (<50) are a blessing vs. the blah 1633s burns (flat <10 PI up to 2nd layer, then peaking up to 250 dropping sharply down to <40PI and staying there to the end).
Here, the better laser optics show through and you can tell with the PI/PO scans from both.
Anyways, it's not to say the 1633s can't do a good job at it - it can with the right media. But I'd rather not depend so much on always knowing exactly which media I'm using & post-testing each disc to make sure they work. Here, Pioneer/Plextors have a better time at producing a solid disc given most media.