Used this build my Wifey onboard air system and it works fine but the mounting holes are very small. They weren't usable in my situation.

Used to control vacuum pods for my CNC.
Will recap what most people has already said. Yes it is small, and to use all of the ports you must get creative, in my case four shut-off values would not fit next to each other. Had to add extensions to two of them to make them fit. Not a big deal, but added to the overall costs. (thus I took off a star)
The fitting are loose in some cases, if you use a tapered tread, not a big deal, otherwise use extra wrapping. I found a thicker pipe wrapping that helped here.
The 3/8 capped ports on the end. No, you are not going to loosen them by using brute force. Not going to happen, so do not try. Instead, hit it with a torch for 60 seconds or so, to break down the sealant around the treads. The plug can then easily be removed.
Mounting holes are EXTREMELY small. luckly I mounted in on wood, so I found a nail the exact size that fit. Would have been an interesting hunt to find a screw that would have fit.
As I said, I am using it for a vacuum control, and with every valve shut, I am seeing no leakage. So a challenge but works.

It has a total of THREE (3), 3/8 inlet/outlet ports in addition to the 4 1/4. This is GOOD, but the description needs correcting.

It comes with 2 3/8 plugs screwed in WITHOUT any sealing tape. They will leak without some. It would be customer friendly to have these threaded with tape. It was leaking, so I naturally suspected my work, it was 15 minutes wasted that wasn't needed.

Using this for an on-board air system in my vehicle. I asked about the size of the holes, but never got a response, so I took a chance. Be aware that the mounting holes as shown in the drawing are NOT 3/16" diameter. They are actually only about 7/64" (smaller than 1/8"), so a #6 screw will not pass thru them as the item is shipped.

I did some measuring as I needed to mount it with some #8 screws (would have preferred #10s), so I chucked it in the drill press and stepped up the hole by 64ths until it was a 11/64", just big enough for a #8. I wouldn't attempt to do this with a hand drill, you might penetrate into the air passage bore.

Will update after I get it mounted and the system operating.

I like the block, it works well and has plenty of ports for peripherals like air chucks, gauges, and air lines. The fact that it has mounting holes predrilled is also nice. Only issues I have had with it is some of the ports were a tad over sized and the mounting holes fit a very strange sized bolt. Nothing a little drilling out couldn't fix. I had to add a lot of thread tape/Thread sealer to get it to seal up and not leak. I had a few fittings completely bottom out in the block so it was a bit of trial and error to get it work right. All in all I would recommend the product for the price that its at. I'm happy with it.