I have a Ridgid 13" planer and I love it. It comes with double sided blades so if one gets nicked it doesnt mean you have to get a new set of blades. I also manually sharpen mine with a diamond stone and a jig and have had no real issues. If youre sharpening your own blades though you have to check that they are all the same thickness. By thickness I mean the "height" not the actual thickness. You also have to make sure its a uniform "height" along its length. If one blade is taller than the rest, that blade is doing the bulk of the cutting and the end result is not so great. That blade will also dull up faster then you have 2 sharp blades and a dull one if you have a 3 blade planer and the finish can be sketchy.
One thing also is grain direction. Sometimes you will run a board through and see ripped fibers and pitting etc. Try reversing the board and running it through the planer the other way, sometimes this is the problem. The same thing happens with hand planes. As far as knots go, they are a pain especially in really dry wood. My planer gives a really nice finish though, but it is in no way hand plane smooth or card scraper smooth.
The planer is great for what I use it for and it has a decent warranty.