I have this scanner. In fact, I have about a half dozen of them, each connected to a different computer. Quality is great... on par with a Nikon scanner according to some tests or reviews. Nearly equal to a Nikon scanner according to some other tests. The main disadvantage is that it is slow. Using SCSI helps. That's a bit tricky if you have a windows 10 computer because it isn't easy to install the scsi drivers under windows 10. There's a trick to doing this. I don't recall the details, but I think it has something to do with turning off some kind of security feature in windows 10 temporarily while you install the scsi card. Anyway, I succeeded in doing this.
The slowness of the this scanner is somewhat compensated for by the fact that you can batch scan up to 6 frames. Anyway, just do the scans while you are doing something else with your computer, and the slowness of the scan is not as bad as you might think.
Slide and film holders are not easily available, so don't lose or break yours, and don't buy a unit without the slide and/or film holders because you can't use the scanner without them.
The lack of being able to use IR dust removal when scanning kodachrome has been mentioned. I have actually succeeded with this on some slides, but at the very least it is not reliable with kodachrome. In that case you would have to rely on other methods of dust removal, such as manually doing it in photoshop. I actually have another way of doing it, but I'm not prepared to talk about it here.
As others have mentioned, plan on running the scanner with Vuescan. It will cost you a little money, but it's worth it. It can be a little tricky to use however. Another option is Silverfast. Rumor has it that it is even trickier to use than Vuescan, and it's expensive.
No matter what scanner you use, be prepared for a steep learning curve before getting good results. At least that has been my experience.
Unless you have a pretty good digital camera, scanning your slides and negatives with something like a FS400US will probably have higher potential to give you the best results, at least if you are planning on scanning with a single shot of the camera. Taking close ups and stitching is another way to use a digital camera, and it has potential for higher quality than single shot scanning with a digital camera, but this method is very fiddley to use.
My suggestion? Stick with the FS4000US. It gives you the most quality for the money (especially if it's free.) You are only likely to beat it with a Nikon scanner, and maybe not even with that, or with certain high-specification Minolta scanners, but the high-specification Minolta scanners of the hard to find and some say the reliability is questionable.
Here's a link that shows some comparison scans between a canon fs4000us and a Nikon LS-9000. You have to scroll down the page a ways to see the comparison.
https://www.photo.net/discuss/threa...yssey-nikon-super-coolscan-ls-9000-ed.495265/
When I look at the comparison scans what I see is that the canon scan was a little bit higher resolution but also a little noisier. However, the scans were quite similar in overall quality. Other tests might give different results.
And here's another comparison.
https://www.photo.net/discuss/threa...00-ed-ice-and-canoscan-f4000us-part-2.496955/. I'd say the conclusions are similar in this case as well.