I am a hobbiest who gets only a month to month and a half a year to really do serious WW'ing.
A few years ago, i bought the 850. I found about the only thing I was using it for was to flatten short chunks of wood I had squared up with bandsaw. I could get a flat face by using winding sticks to line up the cuts. Sometimes, I still had to finish off using hand planes. Not being experienced enough, i did not feel comfortable using the toy as an edge jointer, so it pretty much ended up as a great dust collector.
A year ago, i happened to stop @ The Tool Nut and saw the 850 set up on the frame with fence. i played with it a little bit and the darned thing stuck to my hand so i could not let it lose. It ended up following me home (the frame and fence) where I found it to be great. I now leave the 850 with frame and fence set up permanently. It gets used on nearly every project, especially since I buy mostly rough cut, unsquared and unplaned lumber. The fence is adjustable to any obtuse angle. It is one great little tool.
I have used larger jointers from a little Rockwell/Delta 4" to a larger 8" or 10". For longer lumber, I think I would not be as thrilled with the 850 set up as I might be with a much larger machine. for my uses, it is fine. About the single blade, the blade is skewed so it makes as fine a cut as any jointer i have ever use. The blade comes as a unit on a drum. I have not had to change, but i think the blade is removable from the drum. (not sure on this) For me, i would just get a new drum with blade and replace, sending the old setup to shop for sharpening. With the carbide that goes into Festoys, i don't think i will ever wear out or dull the blade.
I have just recently taken a dose of cool-aid and brought home a CMS. The fence and table are much longer than for the 850, so I can probably handle much longer lumber with that. The fence is adjustable with the CMS. The table is adjustable for the 850 bench setup. Either way, the adjustment is extremely accurate. With the bench model, there is only one adjustment to be made for jointing: the table. With the CMS, you have fence and router bit to set up. I have not set up me CMS yet (lota moving and reorganizing of my shop first), but i do not think my 850 will become neglected. I think the fence on the CMS is adjustable only vertically while the 850 fence is adjustable at any angle from 90º (vertical) to 45º. I wood not feel comfortable working with the 850 fence set at 45º for anything longer than maybe 3 feet. Longer can be handled at 90º. I have never been comfortable jointing any length over 36" at 45 with any jointer. i am sure others with more experience would be comfortable handling much longer lumber with the 850.
For site work, i would have bought into using the 850 had they been around when i was in construction (for "special" customers, I did occasionally do some not too complicated cabinet work. ) I think it is a lot better tool than the old Delta/rockwell I used back in the mid '50's. Maybe not as satisfactory for heavy work as a larger machine might be/have been, but it is a do-able machine.
Tinker