Often spinoffs to a series are of little interest or downright pointless. But this is in my opinion the real ending of Nana to Kaoru--the best order to read in is main NtK first, then this to finish off. Both because chronologically it is considered to take place slightly after the main series, and because it is in my opinion the best sustained piece of NtK. Both the romance and the SM are stronger here than in the main series; the characters grow and it's just absolutely sweet. Nana to Kaoru overall is a really wonderful series; the SM is sexy but it's the characters and the beautiful romance that keep you coming back. And it's worth pointing out that the SM and the romance are not in opposition--to the contrary, this is truly an SM romance, where the sexy restraint fuels and is fuelled by the tender feelings.
The main series has the occasional annoying arc or chunk of filler, but Arashi is pure gold.