The 615 is overall a well executed piece. And you mentioned the Rolex Deep Sea, which is a good comparison regardless of the size difference. The main reason why I like Panerais generally more than the Deep Sea is that the Deep Sea has a really small bracelet compared to the rest of the watch. It makes the watch less comfortable to wear. Panerai watches are always huge, but the strap is also extremely wide, thus spreading out the weight and balance of the watch more comfortably over the extra width of the watch strap. This makes the Panerai much more comfortable for me. The IWC Big Pilot is another example of a huge watch on a skinny strap.
Here are some reasons why I think the 615 a great piece:
1. The ceramic detailing is an interesting touch. Makes the bezel stand out a bit more. However, comparing the 615 with the similar titanium version, I don't see significant advantages of having the ceramic, except you'll have less visible scratches on the ceramic bezel.
2. I really like the use of the different color lume on the dial and the bezel. Blue and green combination looks great in person and has a practical benefit as well.
3. Panerai does not advise the use of the chrono pushers while underwater. And they've supplied screw down buttons for the chrono to absolutely ensure water resistance.
4. This watch has a huge presence on the wrist! At 47MM, it's large, and the LUMINOR crown guard makes almost every Panerai look larger than it is. But having the chronograph buttons on the other side of the crown adds visual size without the bulk. A bigger looking watch that doesn't look any bigger.
It's a really cool watch!