I have the smaller 9911, which has been worked pretty hard and so far is going strong.
A few points:
1. Whatever finish is on the boards will clog the belt before you sand past it. Don't consider trying this without DX, it'll just be a waste of belts. You can clean them well with crepe rubber (either a block or an old gym shoe sole), and I find I need to do that frequently if whatever I'm sanding is at all gummy.
2. Extra pressure basically just seems to clog the mahine faster, and doesn't give a better or quicker result. It's worse with the big orbital I have but still true of the belt sander.
3. I've run 40 grit, but the belt is so coarse and stiff, it cut through some of the plastic surrounding the belt! It would probably be better with the bigger machines, but still take care. Makita belts are much better quality than the cheap ones, and I can make them last for ages with DX and cleaning.
4. Yes you can track the belt right to the edge of the machine (it's an easy adjustment), but that has two issues: (a.) you can only sand on one edge and in one direction, so for example, you could do one end of a window board (in the reveal), but not the other, because the handle and cord and DX hose/bag get in the way. It's the same with floor corners - you can run along most of one edge, but you cannot properly get into corners, AND the sander is determining the direction of sanding, not the grain of the wood, so the results may not be nice. (b.) It would be very difficult to set up the machine so the belt sands to the edge, but doesn't project on top. It's easy to catch walls, skirtings, etc. with that bit, which acts like a rough saw. You can do a lot of damage in a second or two before you notice (DAMHIKT). The "loose" edge also makes sander marks a lot more easily, so needs great care in use -- I like the wedge-shaped front handle much more than I would a knob, because I get more feel (I mean I notice if I'm tilting the machine unintentionally).
Allow for about 3/4" or more (and for the length of the sander, plus a bit) that you can't reach in most returning corners - the bigger the rollers the wider this gap will be.
My Makita often gets used on plaster and other horrid jobs. It's reasonably easy to strip and clean , and having seen it dirty, I realise I need to do this a lot to keep it reliable. But it's self-tapping screws into plastic, so sooner or later, strip/clean will become impossible as the screws will no longer hold. I assume the "professional" models you mention have properly tapped screws, so you can get at the airflow channels and round the roller bearings and the motor - you need to (the other reason is that old dust can contaminate other work later). I'd want to know this before spending the extra dosh. I'd buy another one like mine, but I'm not at all sure I'd splash out on the bigger machines without finding out.
So I think I'd hire to do a floor!
Has anyone used a sanding frame, and if so did it help? They're a silly price considering what they are, but I've always wondered...
E.