The reason I started with smugmug, and willingly pay good money for it (for that price I could get an extra box of 20 sheets of 4x5" Velvia 50 a year, even at the new prices), is that I like the customisability. I hate flickr with its white backgrounds, I wanted black.
I like smugmug's ability to change almost everything, make galleries look different from each other, all of my public ones (as empty as they are, I should upload more) look the same title-bar and all that, but some privately-shared ones look different, I've stripped all identification and links back to my public galleries.
I left Flickr because of their horrible security (I'm well aware that no site online is 100% pirate-proof) and their horrid site "revamp." I paid for a Pro SmugMug account to sell my pictures as well as because of the great customization options. Taking that one step forward, I went so far as to have a custom template designed by one of the many SmugMug customization firms.
All in all, I LOVE SmugMug...the way it is.
As many have stated/alluded to, I'm concerned that the code utilized by my custom template may not jive with whatever SmugMug has up its sleeve. I don't want to access my site after July 30 and see that I've been essentially bricked and will have to either A) Pay the Customization people more $$$ to make my site compatible with the new SmugMug, or B) leave SmugMug altogether.
Now, if this update will not alter custom functionality as it is, but rather provide new (optional) enhancements, then I'm all for it, and I sincerely hope that's the case.