As a continuation of the first series, you can expect the OVA to fill in much of those undesired loose ends and plot holes that cropped up throughout the series. These episodes focus on Kurapica's quest for revenge and his eternal hunt for his clan member's eyes. Fans of the original series will note that Hunter x Hunter is not a terribly fast-paced anime, and tends to build the story around the characters and their interactions with one another, rather than with action. It lures the audience in by showing them all the interesting depths of the characters -their goals, their dreams, their struggles- and makesus root for them when they hit dirt. This OVA does so much more than that. Following up on the hints we were given with the original series, the OVA gives its audience a much deeper look into Kurapica's emotions and hardships, while also managing to show the heartfelt friendship between him, Leorio, Gon, and Killua.
Kurapica's character, I feel, was for the most part just a supporting character originally and wasn't given too many layers that really fleshed him out and gave the audience much insight into his emotions. Suddenly this problem is completely rectified: he becomes so much more of a main character than I thought he could be, he shows new and interesting sides of himself that I didn't know existed, and he draws you into loving his character so much more so than before. Furthermore, the OVA builds on his friendships and shows the trust, loyalty and support between them.
The Spiders are given stronger personalities and I feel they are much rounder characters than they were before. Each one has a unique relationship with each other and with Kurapica, and you really start to play favourites as he lays out his plan to dissolve the organization.
All in all, for me character was a 10.
The story is these episodes is a bit more fast-paced than what fans are used to (which is a given considering the number of episodes). However, this is what I found so exciting in this OVA. There's a lot of mystery in this plot, and Kurapica's planning skills with his limited number of resources is superb. Just when things start to fall apart, he critically analyzes the situation and pulls everyone through the trouble. I love all the problems Gon and Killua run into, and those two pulled a few stunts to get through them, even surprising many of the Spider members. I must say without a doubt the story was a 10.
As far as art goes, I didn't notice a whole lot of difference between the OVA and the original series. The lines could be cleaner, but that's probably just my preferance rather than the quality of the art. To me it wasn't bad: 8. For sound, in terms of music: it wasn't spectacular but it wasn't terrible either. The opening and ending themes definitely aren't the best, I would say they were average. Background music and sounds were acceptable (but it isn't shinobi battle music, if that's what you're expecting). 7/10 for sound.
To summarize in one short sentence...
This OVA is everything you were expecting and more.