Aftermarket SPD-SL cleats Given that original Shimano SPD-SL cleats are not the cheapest, and do wear down quite quickly, looking for cheaper alternatives.  Have seen various aftermarket options for half the price.  Has anyone any experience in using these?
In the automotive industry, &quot;aftermarket&quot; refers to, from Wikipedia, all the secondary market for all vehicle parts and accessories, after the original sale, including replacement parts and accessories, whether made by the OEM or not. I don't frequently hear this term used in the bicycle industry, but it does apply.
Licensed copies
In this particular case, it's worth discussing patents. To my current knowledge, Shimano's SPD-SL pedals are still under patent. I see on Google that Exustar offers an aftermarket version of the SPD-SL cleat, although it has 4.5 degrees of float (vs 6 degrees for Shimano's yellow cleats, likely not a limiting factor for most riders). I believe this is a licensed version of the SPD-SL cleat. I have no personal experience with it, and I'm not sure how these licensed versions have been sold.
Exustar have made either licensed or slightly variant versions of Look cleats for some time. Cyclingtips' review of road pedal systems in general argued that Exustar's Look cleats were better in some ways than the stock ones in some ways. Clearly, Exustar know how to make a decent cleat in general. I suppose you could also look through the Amazon reviews for their SPD-SL equivalent cleat. In general, licensed copies should be OK. I would expect them to be equal to or worse than the original product or OEM version.
In cycling, I can't really think of a licensed copy of a product that's superior to the original. In some cases, rim brake pads come close: Kool Stops are noticeably superior to many stock Shimano pads, especially the low end ones. At the high end, I've heard it argued that the Swiss Stop BXP is superior to the stock Ultegra or Dura Ace pads. These pads use the dimensions of Shimano's brake pad holders, but I would not say these are really licensed copies (the pad compounds are different).
Xpedo and Exustar may make licensed versions of the Look Keo pedals and cleats, and these products may vary slightly from Look's design. It seems like they may not officially call their systems inter-compatible with the original Look Keo pedals or cleats. The Favero Assioma power meter pedal is built on Xpedo pedal bodies and cleats, and a different article describes those cleats as a Look variant. The same is true of the Powertap power pedals. Some users report that original Look Keo cleats fit the power pedals, but the fit may degrade over time. Garmin's Vector power pedals (the second link) appear to come with Look Keo cleats, and if so, then I would assume Garmin licensed the Keo design from Look. I'm not clear on the licensing status for Xpedo and Exustar, but given the designs appear very close, I would assume those firms had to pay licensing fees. John Weirath, a bike fitter, documented on his Youtube channel that the variant/licensed Look cleats may have some dimensional differences versus the original items. In that video, the difference was enough to require a different cleat position.
Un-Licensed copies, i.e. counterfeits
Dishearteningly, there may also be counterfeit SPD-SL cleats in circulation. One post on the Paceline forum, this blog post, and at least one person in this thread on the Bikeradar forum reported bad experiences with cleats that were suspected counterfeits (i.e. advertisement implied that they were Shimano cleats or didn't clarify that they were not made by Shimano). Complaints related to durability and materials quality. However, at least one person on the Bikeradar thread did report satisfactory experience.
In general, if you are buying components that you know to be knock-offs, especially illegal knockoffs, of a genuine product, I would assume that the materials may be poorer quality. This could mean that the plastic is less durable and may break or wear faster, or that the rubber bumpers may not adhere as well as the stock cleats. Also, the tolerances could be poorer, i.e. you may get a set of cleats that fit too loosely or too tightly in the pedals (i.e. the variance in the physical dimensions is higher than the stock cleats). Naturally, it's not a given that non-OEM parts in general will have substantively poorer quality and function than OEM parts. It's just that it's more likely with parts that are counterfeit.
Also, apparently you now need to be aware that Amazon and eBay may have counterfeit SPD-SL cleats, as well as other counterfeit items. I hadn't really known about people selling counterfeit SPD-SL cleats until the Paceline post. I think that knockoff may be a synonym. This refers to something that's just a direct copy, with no regard for legality.
I recall that eBay had a problem with fake Oakley sunglasses years ago, when I was more credulous (and yes, I got scammed). I ordered a set of CPAP filters on Amazon. They weren't OEM, and it was clear that they fit a bit looser than OEM filters. My opinion is that with a relatively low-cost item like a replacement cleat, I am not likely to bother with third party options unless I know the third party produces good quality. This would actually include Exustar's cleats, so I may try them next. Items not licensed by the manufacturer are out for me.
Terminology - more on knockoff vs. counterfeit
Above, I refer to suspected counterfeit Shimano cleats because I believe that many sellers are passing off their products as Shimano, and I don't believe they have the legal right to do so. It's generally possible to produce a knockoff product without violating the law. If Shimano's SPD-SL system is still under patent, I assume the patent covers the physical dimensions of the cleats, so it's probably impossible to legally make knockoffs that actually work.
Shimano did, however, let the patent on their SPD mountain bike pedals and cleats expire in early 2019. SRM produced a pair of power meter pedals, reviewed by DC Rainmaker, that are compatible with SPD cleats. I don't think people would instinctively call this a knock-off product - this is a completely new pedal design, and Shimano don't even offer a power meter pedal. In any case, Xpedo either produced SPD-compatible MTB pedals under license, or else they just started producing them in 2019.
Legal knockoff products do occur in cycling. One example of legally copying a product is the Rock Brothers version of the Spurcycle bell. Spurcycle failed to patent their bell, so Rock Brothers was able to legally copy the design - although, as I mentioned above, their version had noticeably worse performance due to poorer materials and construction. Spurcycle is a small company, and they simply did not know about the importance of patenting. I'd assume Shimano didn't forget to patent the SPD-SL system, or to renew the patent if they were able.
Alternatively, in 2018, DC Rainmaker reviewed the Magene Gravat 2 smart trainer. He has extensively reviewed smart trainers, and the Gravat appeared to directly copy the physical dimensions of Wahoo's Kickr. He described it as &quot;a bit of a copy-cat trainer.&quot; In this case, though, there is also the internal engineering of the trainer (which has a flywheel, a drive belt, and various other mechanisms), and there's also engineering the software to drive the trainer. The trainer as a system may not be close enough to Wahoo's for them to raise legal objections, or it may be impossible to fully patent every aspect of the trainer (IANAL). Also, Magene's color scheme is clearly different from Wahoo's, so there at least should not be any confusing Magene's product as a Wahoo.
Less closely, Silca recently released a compact saddle bag called the Mattone. Some commenters on the Bikerumor article I linked accused Silca of ripping off the Velocolour Rocket Pocket. IANAL, and I don't even know if Velocolour applied for a patent here, but the Silca product may be different enough to not be considered a copy - for instance, the attachment mechanisms are totally different. In the fashion industry, brands rip each other off frequently. For interested parties, here is a Vox article discussing the existence of the phenomenon, and the public policy case both for and against stronger copyright laws.