ISO 200: Smooth Tones/Contrast
ISO 400: Good Snap like pushed Tri-X
ISO 800: Very Contrasty (Personally I would push at this speed)
(ISO 100 & 50 is really too flat, but usable with adjustments in PS or LR. This speeds will also yield the finest grain. Beyond 800 we recommend pushing, or Delta 3200)
According to Ilford's own data sheet, XP2 can be exposed from ISO 50-800 without modification in development. In fact, this is a critical way to understand how to get the results you want. Because C41 films handle over exposure so well generally, you can really modify contrast through exposure. Shooting this film at 200 gives you those remarkably smooth tones and highlights, with an excess of shadow detail. They're the kind of negatives you have to work pretty dang hard for with traditional silver gelatin films (in my opinion). As you go up in ISO, you tighten up that contrast to a significant degree. ISO 400 has good snap, and 800 is even more contrasty. Personally I would recommend pushing when shooting this film at 800, or 1600. When shooting this film at ISO 100 and lower, expect to have to do a bit of work to normalize the low contrast results, but they will still be quite usable.
Ilford does not recommend pushing, but I have done it and it can yield good results, though not passed 1600, whereas a film like HP5 is great up to 3200. We also do not recommend using expired XP2.
It should be noted that all of the images in this article were shot with simple metering techniques, such as in camera metering in the Hy6 Mod 2, or a handheld incident meter in the Hasselblad or Rolleiflex 2.8E. XP2 is simply a forgiving film, and you don't need the zone system to get beautiful gradation across the available range of tones.