Yeah, if you cook a giant piece of meat for three hours at 145, you get the whole thing medium rare and you just brown the outside. I've seen it suggested to brown first, then you get the caramelized juices marinating the meat while it cooks. You can marinade in the bag, but I use a finicky vacuum sealer and liquid in the bag before sealing is a bit of an issue. Using the zip lock method is better for marinade-in-bag cooking.
The thing is a bit of a pain in the ass because you need such a large water vessel. But once you have the gear and the technique, it's amazing to eat beautiful, medium rare pork roast that spent a few minutes in the oven or on top of the stove browning. I tend to do big batches and season afterward, but I could just as easily (but with more wasted plastic) do multiple smaller pieces at once, each in their own marinade.
I highly recommend.
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