After a few weeks of using the ES20 for recording sports (mainly basketball) here are my thoughts:
Let me preface this by saying I am not an expert, these are my observations from using the ES20. (See the original post for more on that).
For basketball, I noticed two types of artefacts:
A) The most common is when the camera tries to follow a player after a made basket as he is running back on defense with the background being the crowd in the stands. What macroblocks mostly is the crowd in the stands, which is of no particular interest anyway. But nonetheless some people find it annoying, and others want to have a clean copy. This is sometimes observed in SP mode as well but it is much less pronounced than LP.
B) movement-related: sometimes during fast movement of players. As expected, this is more common in the 4:20-4:50FR mode and rare in the SP mode.
The artefacts seem to depend on various factors -in my estimation-, including:
* mpeg compression and ES20 internals
* quality of the original broadcast (eg compression) - some instances (of digital cable channels particularly) already have light macroblocking in some of these situations
* camera and cameraman handling and position, camera movement, lens, camera angle, etc
* color/lighting in the arena (in some arenas you may notice jaggies in the 3-point lines or the paint lines)
Recording modes and Macroblocking
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LP mode seems to have more macroblocking and artefacts than SP or FR (4:20 to 4:50) or so. EP mode (6h or 8h) is unwatchable for sports because of the compression/smoothing/etc. 3hr-FR seems to behave similarly to LP mode. SP mode is better but not 100% perfect. I haven't used XP much so I can't commend on that one. However the ES20 does not deserve all the blame because I notice lighter compression artefacts in the original Comcast DVR picture and DVR recordings. In other words, the additional compression of the ES20 may be magnifying the side-effects of Comcastic compression. But regardless of that,the final picture is what matters to the viewer

Other important factors to consider
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Another thing to keep in mind is what exactly are you planning to record, and what sources, as different dvd recorders may be more suited for some things better than others.
In my case this falls into two main categories:
1) Active watching and recording. (chase-play live games, live live games, already recorded games or even VCR games). For most new games of the 05-06 season, I record them to the Comcast DVR, then start watching after 30 minutes or so (for chase play/commercial skip), and at the same time record them to the ES20 while watching. This has the benefit of removing commercials since I don't watch the commercials anyway. It only has the additional overhead of pressing PAUSE before and after each commercial break. Not a big deal.
(Btw, most games fit on one disc at SP with room to spare. But if it's a long close game with lots of whistles/delays or it goes into 2nd overtime, you may fill up the disc).
For this type of recording the ES20 has the disadvantage of basically two options: either SP or LP. You can't pause/restart recording with the FR modes. The Pioneer MN-32 mode would give a lot more choice here!
2) Passive recording (eg not watching): This could be either from the RF tuner, cable box, or VHS. FR can be used here to utilize the most out of the disk since the size of the recording is predetermined for non-live games, and can be guestimated for live games (eg 2:30 - 3hr FR)
* I am not doing any PC editing nor do I wish to do any post-recording editing in the device (I would do some if DVD-RW(VR) was available, or if DVD-RAM was as cheap as DVD-R/RW, but as it is right now, there's not much to gain from heavy editing, other than commercial removal which I personally find a hassle unless I am watching/recording at the same time in which case commercial removal is easy)
* For VHS recordings the ES20 does clean up noisy tapes. Don't expect miracles but it can turn an unwatchable tape into a watchable one in _some_ instances. YMMV and it also depends on personal preference, equipment, etc.
6hr VHS or 4hr LP mode
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I think this is one of the biggest trade-offs of both this device and deciding whether to go with DVD or continue with VHS tape. Each has its pluses/minuses, which have long been debated so I won't get into that debate. Neither one is perfect, so it is up to YOU to decide what to use and when. Obviously future generation DVD recorders will improve, so perhaps a hybrid strategy of VHS and DVD at this moment may be one possible solution for those who can't decide between the two.