I think the PS1000s do improve greatly with burn-in, although I liked their sound from the get-go. Mine seem to excel in particular with percussion and bass definition - great snap, beat, and texture. I really love hearing drums on these cans (and I never was that much of a drum fan before). With time, the PS1000s have also become better resolving and fairly even from top to bottom, though the cans are definitely more weighted toward the lower mid and bass than my HD800s. But then, these are Grados! Along these lines, luckily, I haven't really experienced any sibilance issues. Interestingly, if you press the cans to your ears and keep your hands on the outer cups it greatly affects the sound, but in a very bad way - the cans then become shrill and thin sounding. Oddly, with the GS1000i's, this had the opposite effect - doing so would give me more presence (due to the recessed mids) and a slightly improved sound.
Overall, I also find that the PS1000s have good sonic weight. While the PS1000s don't have the wide soundstage of the GS1000i's, they do not feel confined - rather, you get a nice sense of where the musicians are located and also fine definition - I'm able to hear and place individual instruments, regardless whether I'm listening to a rock or jazz group or a large orchestra - it's just all at a somewhat smaller scale than with the GS1000is.
That said, as with most really good headphones, they are very revealing, particularly of any artifices in the recordings themselves. Example: I was listening the other day to a soundtrack to the Indian film "Dharm," using Media Monkey, and the PS1000s clearly showed that the recording was a patched together sound engineering job - almost unbearably artificial. By contrast, listening to The Allman Brothers at Fillmore East (Im Memory of Elizabeth Reed) or the Grateful Dead Europe '72 (China Cat Sunflower/I Know You Rider), or Bob Marley Is It Love Remixed (off the Legends Deluxe Edition), or some really nice high-bit downstream stations (jazz and classical), the PS1000s really shined - all those sounded amazing with these cans, allowing me to visualize all the players and instruments - clarity, definition, resolution - superb.
In short, you should definitely give the PS1000s a little bit of time and they should vastly improve; hopefully your dealer has given you a 30-day return option, so I would take full advantage of that. Finally, I've noticed that the improvement, while constant, comes in fits and starts (kind of like losing weight - some days it seems like you've reached a plateau, and then, after a couple of days, you're back on track) - a little patience definitely pays off.
Weight-wise, the PS1000s never bothered me, though of course the GS1000i's are feather light. The weight, as I think Zanth mentioned way back when, is distributed to the sides, so hopefully you'll get used to them (I have a smallish head, so they don't sit very snugly on my head, but that doesn't bother me, and maybe also makes the cans' weight less noticable). As for the physical finish of these cans, I couldn't agree more with all the other posters -at $1700, this is really inexcusable.
In the end, though, despite this issue (and I agree completely that it is a major one) it's still the sound ....