I think that the HL850 EB planer is possibly Festo's best tool ever. There's certain tasks it can perform that simply aren't possible elsewhere. That "unlimited" rebating capacity is probably its best & arguably most useful party trick for the production of rebated frames, adjusting of the seats of non-adjustable hardware such as aluminium windows in their wooden frames, sills, reveals, weather rebates etc. Solving the difficult problems of first fix carpentry in a manner with which any lesser tool would struggle, if not find impossible.
Secondly, the slicing action of the cutter means that a sharp blades' finish is sufficient without much if any at all subsequent sanding for painting or finishing prep. It's sheer bulk, width, length & heft also seem to work well to provide a smooth, balanced and linear action. All lesser tools tend to leave a distinctly rippled finish, its "frequency" dependent upon feed-speed.
Its ability to quickly, easily & after some practice intuitively adjust the depth of cut "on the fly" midcut is exemplary & (as a consequence of the front "throttle" twistgrip) possibly unique. In fact just about all mechanical controls & adjustments are available within reach of one's fingers wrapped around the front twistgrip: DOC adjuster & rebating cutterblock endguard retracting lever (which can be hooked up once the superstrong return spring stretches a bit). Sure signs of clever, intuitive design.
Great for right-handers, but nigh on useless for the cack-handed minority unfortunately. Therein lies some of the tool's inherent disadvantages, too. Far from being a site plane to be used easily one-handed for easing rebates and tight fits, tempering arrises etc., this tool is simply too big for many of the multitude of tasks required of a site planer. A firm grip is required with both hands, & the tool can only realistically be used in a horizontal mode, flat on, to a safely clamped or restrained workpiece.
Its single-sided cutterblock support is especially dependent & hard on its dual-bearing mount. It's not at all unusual for these bearings to fail, requiring regular replacement in heavy duty use. Luckily, these are easily accessible & exchange much more easily accomplished than with more conventional dual-suspended cutterblock bearings. These bearings should be regarded as mere"consumable" parts in the same manner as blades, carbon brushes & drive belts are.
The electronic idle control, as also fitted to the now discontinued top 'o the line Metabo, provides current-induced "acceleration" under load, making for smooth cutting performance, and the electronic brake (my own model predates this innovation) adds an additional layer of utility & safety.
Ideally, the tool really needs to be partnered with a more "conventional" one-handed planing tool for optimum site utility. My personal faves are Metabo's beautifully tiny little HoE 0983 or the extremely useful 4" cutting capacity AEG/Atlas Copco/Milwaukee HB750/HBE800 clones. All of these are now unfortunately discontinued, but there must be hundreds of thousands of these tool still available in garages, builders' toolkits & workshops all over the civilised world. The sheer quantity available second hand attests to their former popularity & ubiquity.
Makita still make some excellent trade-quality small planers that would partner well with the big Festo/ol. The Festo/ol planer still soldiers on in relatively unchanged form since the original HB750 machine was released in the early 80's. That sort of longevity almost rivals the Elu/DeWalt routers' claim to being if not the the perfect then at least the definitive tools of their type.