So I 1) trussed and cut off ends 2) let it sit at room temperature for half an hour and 3) taken it out early and let it sit in foil to continue cooking a bit longer.
It was markedly better but still not “good”. I don’t think I like the cut to begin with 
Thanks all for helping. Back to shoulder for me!
That’s pretty much what I think of pork tenderloins, unless they’re either stuffed with something amazingly yummy, or pounded flat, breaded, and served on a bun. And even then, they’re still shown up by a good pork shoulder.
It is kind of a white meat vs dark meat kind of thing. I generally prefer pork shoulder, but I enjoy loin when it is jerked, and there’s a pork roast with a salt, cracked pepper, and cracked fennel crust that is great when served with a tart gravy. (This is the recipe, although I add peppercorns to the crust.) You do have to like fennel, though. If you don’t, a simple pepper crusted pork is pretty darned good, too. Or you could go all French-like and do mustard, thyme, and herbes de provence.
My favorite use for pork loin, though, is in cold cuts. (Although I use loin, not tenderloin, for that.)
ETA: Oh, duh, and of course, as Athena says, a pork tenderloin sandwich. Those are awesome when done well.
Get a leave-in probe thermometer (like one of these), put the probe into the deepest part of the loin, and do what you’d normally do in the oven.
When it hits about 138-140, take it out, and let it rest. It should coast up past 145 (the USDA revised minimum for pork), and remain juicy and tender.
Pork muscle begins to denature around 150-160 degrees if I recall. When it denatures the proteins shrink and squeeze out the water resulting in dry, tough meat. There’s not much you can add to the meat to keep it from drying out if it reaches that temperature so the trick is to try and cook it thoroughly to 140-150 degrees without letting it get over 160.
Low-and-slow is the best way to do this. Cook the pork in a crock pot on low with a favorite sauce for a few hours until the internal temperature reaches the desired point (e.g. 150).
If you like to cook a lot consider getting into sous vide. It is a (generally) fool-proof method for cooking meat to the desired temperature without overcooking it. I’ve cooked pork tenderloin using sous vide and it came out great. I’ve had friends say they didn’t like pork tenderloin until they had mine.
Sous vide machines can run $500-600 but if you’re handy you can make one for as little as $50.
I’ll eat pork with a little pink in it, but I’m probably never going to be totally comfortable doing it. There’s always this voice in the back of my head telling me that I’m about to die horribly.
My technique for cooking pork loin: