You also get the added benefit of soldermask and silkscreen. When you consider the cost of your own time, the machine costs, material and of course bits it probably works out cheaper to get them done externally. Perhaps things have improved in the 5-6 years since I gave up on milling PCBs but personally I think you are much better off just paying for one of the fast turnaround fabrication services. Bear in mind these were EU and USA built specifically for PCB work. Broken mill bits are another pain, you sit listening to the machine drone (plus vacuum and air compressor) for hours while you do other things only to realise the mill bit broke 10% of the way in. Even the slightest warp in your FR4 can destroy your results and leave you with multiple efforts. However they often needed a lot of attention to get the right results, particularly on larger boards. +1 to what Mike said, previously I've used LPKF and T-Tech milling machines and sometimes they work beautifully so you are can go from concept to PCB in a single day. I'm not a good enough circuit designer either so I have to build my circuits to make sure they work otherwise I wouldn't bother. However, when I mill a one-off I can do it many times faster with carpet tape. Both methods will pay off if you're milling a batch since changing stock is very fast. I've never done this myself but I heard that drilling/milling into the vacuum holes ( or grooves ) is not a good idea - weaker vacuum, abrasive swarf going into the pump, etc., so you spend time designing your table for the board you're making. Everything can be machined except the horizontal hole which you would drill, tap and screw in the vacuum port.
Another way is to clamp a 1" thick piece of HDPE and make the vacuum table out of it.
The fastest way to make sure is to mill the top surface flat however, since vacuum table is expensive and your machine may not necessarily be good enough to mill aluminum you spend time with indicator in the spindle measuring and shimming. You mount a vacuum table (or any other jig for this matter) on the machine and start wondering if its surface is orthogonal to the spindle.
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