I've used XP2 quite a bit. My best results were when I exposed it as if it were ISO 200 and then upped the contrast after scanning or in the printing. It's a very convenient film to use in cameras that have very limited exposure settings (vintage box cameras, Holgas, etc.), as it has such a wide, useable exposure range, on the same roll, without changing the developing at all (ie. you can shoot one picture at ISO 200, another at 400 and another at 800, if you want, or if the lighting conditions change - and this does really work). I prefer more conventional B&W film like Tri-X and HP5 though. XP2 is great for scanning, because, since it's really just colour print film without the colour, you can use Digital ICE and other similar infrared dust removal systems, unlike conventional B&W films. One downside of chromogenic C41 B&W films might be that you don't get the longevity that you do with conventional B&W - but, I can't say I've had XP2 negatives long enough to know.

Now, personally, after having used quite a bit of it, my opinion is that, if you only scan your negatives, XP2 is great. But if you print conventionally in a darkroom, you may as well use conventional B&W and go through the one additional step of developing the film yourself.