We've eaten in the Informal Dining on the Constellation many times and have received good service. The wait staff, IMHO was a little more "rough around the edges", as daysatsea indicated, it's a training ground for the gents who are moving towards food service in the San Marco (main) Dining.
I agree with Norm regarding the dress on Princess vs. Celebrity, after my 2-week Amazon cruise on Princess. IMHO pax do tend to "dress up" more on =X=.
Sid1962, I hear you about running to Thallossotherapy after dinner. That's why I like the main dining. Nothing like a soak after dinner (if I'm not enjoying the show).