We looked at the Fathom 40, it was one of the boats we considered vs. the American Tug 39 we bought. I thought the Fathom was a very nice boat, well built, a great interior layout - they pack a lot into 40 ft. They have a good turn of speed for a 40 ft boat with a single 425 hp engine, nearly 20 kts. In the end, we decided to go with the American Tug for several reasons.
1) As others have mentioned, no side decks on the Fathom. That's kind of a deal breaker for me, I really like having access all around the boat for docking (there won't be any dancing on the relatively narrow side decks of our AT, but they're serviceable). The interior of the Fathom is so spacious at the expense of forgoing side decks. 2) The hull is more of a full planning design with a small keel (for directional stability?). While we didn't sea trial a Fathom, I envisioned it as potentially not being as comfortable in rougher conditions as the AT, which has a solid, deep full keel and is semi-displacement (not to open that can of worms...). 3) I also had questions about being blown around by wind when docking, with the high silhouette and shallow keel. Another way they get so much interior space is by double-decking, stacking one full headroom compartment (the pilothouse) directly above another (the staterooms) - it gives a huge interior, but with the trade-off of height. 4) The hull is cored. It seems very well built, but my own personal religious preferences are for a solid glass hull. 5) Past experience with American Tug. We previously had a 34 and loved it, so it was admittedly a higher bar for any other brand.
I think the Fathom is a good looking, spacious, well-built, economical boat with a good turn of speed. If most of one's boating will be in relatively protected waters where a planning hull would work, it would seem to be an alternative to boats like the Beneteau Swift Trawler. It seems like it would be a perfect boat for looping, and particularly if you prefer the "beefy" style KJ observed.