Some thoughts, since I can't help myself
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I have 3 Quad 4C's - same machine as RX8, but different software (same mechanicals for the most part). Before these I had a Fuji IP-3. Before that I had two Dynapert chip shooters. Before that I had another gantry machine. Before that I had some customized home-brew setups.
In my humble opinion...
1) I could not agree more with RX8 when he pointed out the folly of spending a lot of time trying to overcome shitty mechanicals with all sorts of software and/or vision tweaks. A quick and precise underlying X/Y/Z mechanical system using the right parts is not expensive or difficult to build using established and accurate parts. There's no reason to be dicking around with substantial backlash and such. You can and should use the software to zero-in the accuracy, but starting with linear rails, closed loop positioning and a rigid structure is a no-brainer.
2) Mike said it earlier and he's right on the money. Feeders. Feeders. Feeders. Everyone ignores feeders and focuses on placement. Forget placement - it's easy. Feeding and picking is the issue. There is no point whatsoever in ignoring the feeder issue and loading little cut strips of tape on the bed of a machine and/or pre-staging parts by hand. You spend an hour setting up the machine so you can not spend an hour placing a board. False economy.
3) RX8 said it right when he said that the machine he would build today, after owning a "real" PnP is not the machine he would build before he actually owned one. That hits the nail on the head. Most of the issues people dote over when designing in their head are non issues and they gloss over the ones that DO matter (feeders!). A useful machine should hold a lot of parts. It should be easy to program. I don't care about speed all that much compared to how much I care about reliability and unattended operation. My machines now let me go so something else. They are not fast by modern standards, but I can place hundreds of boards per day. And even if I am making $10 per board in profit, that would be thousands per day, tens of thousands in profit per week. If I needed 5 times the throughput, I could buy more of these same machines, or invest in a $300k Juki/Assembleon/MyData/Universal/whatever.
4) Forget paste dispensing, IMO. I chased that pipe dream for a while - then I re-learned a lesson I already knew. Things are done the way they are not because everyone doing it is too stupid to think of a better way, but because the way it's done works and works well. Etched steel stencils without frames and pasted on a desk using masking tape to hold boards in place can support multi-million per-year operations. We can paste 500 boards (50 panels, 10 PCB/panel) in maybe 20 minutes. And we can place them in maybe 2 hours. And reflow in about 30 minutes. There is zero economy to be gained with paste dispensing. The only exception is prototype boards or glue dispensing for wave soldering, but those are specialized applications. And even then, given the time to program the machine, I've never had a case where it wasn't faster to just do it with the desktop pedal-operated dispenser.
Vision is what it is - been discussed to death and I think it's well understood and well implemented in most machines. The big discussion should be about feeders.
I've seen all kinds on the machines I have owned - varying levels of complexity and success. I like the Quad feeders the most. They are self contained - basically just power and index input. The feeder handles holding the reel, indexing a given amount, peeling the plastic cover tape, presenting the part, and adjustable index amounts. There will always be some gears or belts involved, but a simple servo indexing the tape with a mechanical linkage tying the cover tape capture to the tape indexing I think is the best solution.
But the big elephant in the room about these projects is that you can't sell a PnP that cost $5k in parts for $10k. You need to sell it for more like $40k. Because when you add in the cost of R&D that you have to spread over the sales, and you factor in the marketing and sales cost, and the cost of support and warranty, you will need every bit of that margin to run a business.
If you think you will open source stuff or have online forums for support - you are limiting your customer base to hobbyists who won't spend too much on the machine, but without a real business and support, you will never get business customers interested.
PPM (the company who took over the Quad line) is not very far from my shop. I've been up there and toured the facility. They charge prices for spare parts, support and whole machines that most people on this thread would consider outrageous. They are doing OK by New Hampshire small business standards, but their offices aren't exactly luxurious buildings of glass and steel. And they get most of their machines for FREE or next to free - and if it's beat to shit or missing parts, they probably don't even want it for free. Then they refurb them and sell them for $25-50k.
Those are the kind of realistic margins you need to be able to sell PnP machines to business customers. And if you sell to hobbyists, it's a very limited market (in size and how much $$ they have to spend) and IMO, probably a support nightmare.