Having just finished the PADI deep specialty, ill share my thoughts.
The theory portion didn't add anything beyond what open water and advanced open water provided, if you have those two books than the deep diver book will just be redundant.
The practical was well worth the experience in my case, We went to recreational limits in both depth and NDL under close supervision, we were shown what our computers would do when reaching NDL and how to ascend to "ride" the NDL to shallower depths. These dives were carried out in the St Lawrence River under less than optimal conditions.
We were also exposed to carrying pony bottles and switching to them.
The practical portion left me feeling confidant in my abilities, which is good because I was a little underwhelmed by the theory provided.