P20 is a somewhat small medium format sensor. So make sure the widest lens available for whatever platform you're on will be wide enough for you:Phase One Lens Visualizer
Resolution is low by today's standards but if it's all you need, then by definition that's not a problem.
High ISO isn't great with these older backs compared to newer backs, but a lot of progress has been made in raw conversion since they were first shipped, so they are significantly better now at non-base-ISO shooting than when they first shipped.
The LCD quality and review-in-the-field options are just "ok" - nowhere near the more modern IQ/IQ2/Credo series or a modern dSLR.
If you're shooting a lot of fabric or repeating patterns you should be aware that you might run into moire (this is a 9 micron back, which are more prone to moire than newer backs).
Still fully supported in Capture One v7 and, judging by the continued software support for every digital back Phase has made, will be supported for years to come.
It would be preferable (warning: highly biased opinion) to have a warranty since at the price any repairs are likely to be a good percentage of the purchase price of the back. The only warrantied P20s at this point would be from a dealer.
Maybe you could tell us a bit about what you'll be shooting (camera platform, types of lenses, types of subjects, tripod/handheld, tethered/untethered).
Color, tonal transitions, dynamic range, pixel sharpness, ability to use with many kinds of bodies, native square format, fast shooting speed. All these make a pretty good case for a P20 even today.