We're a PT based facility, and I use it every day. But after a year of trying to make it work, I went back to Sonic for mastering, as much for the sound as the editing features so sorely missing in PT (e.g. beat-matched edits in PT are much more difficult and time consuming, not even close). In Sonic (and to be fair many native DAWs and Sadie and Pyramix I imagine!), I have "free" gain changes... no matter how many level adjustments one makes in the chain, the file nulls back to source. Similarly, reciprocal EQ changes null closer to source than any TDM plug can touch (tens of decibels better).
While PT can run all or most of my plugs, routes more capably than sonic or most mastering daws, and never runs out of gas (dsp), in the end the same settings sound different (and to my ear "worse") than sonic or even Logic. This isn't a slam on PT: It's optimized and designed for multitrack work, so different compromises are made. As much as I dislike it for mastering, my multitrack sessions have been a joy due to it's features and power. I "came up" with Logic and later DP, and keep current on both, but over time I've found I prefer PTHD to both when under the gun. Different tools for different jobs I guess...
-d-