I just finished a trip on Sapphire (Aug 1-8) and had a great time.
I used PC dining and had no problems either. I really like the option of going where you want, when you want and sitting with who you want. We werre there for a wedding too so the flexibility PC dining afforded us was well worth it.
I used room service almost every day (ate at horizon's once). There was the occasional glitch but it was just a matter of letting them know, what you wanted and things got resolved. Staff really seem to have been trained to make the customer happy as we did challenge them a bit and they always rose to meet my request (with one exception, when I asked the Captain for a tour of the mechanical room he politely declined).
Generally speaking people dressed up for the formal night though a good percentage did not. Probably 30% in Tuxes, 30% dressed semi formal, 20% dressed business casual, 20% dresses casually (many of these people did not go out much so you only saw them in the hallway when you went back to your room).
For excursions I did the heli tour in Juneau too. I would definitely not use Temsco though, unless you want a bunch of rude kids telling you rudely what to do. Go with Coastal and arrange it yourself. Having said that, I also took a cessna 185 over to glacier bay out of skagway. That was cheaper and better than the heli tour. I used Mountain Flying Service and really enjoyed it.
To Fran, I think dolphin or caribe are best as they are central. The ship is big enough that is doesn't matter where you are (front, middle, back) but more important to be close to the people you are spending time with. Just get the best room you can and worry about location later.
I brought a bunch of alcohol onboard. Just make sure you put it in your checked baggage and not your carry on.