Its still too early to tell. From early results, he has soft vocals like Sonika. He is much clearer than other Vocaloids with soft vocals like Luka and Sonika, but not as clear as Big Al. Though he isn't as easy as Ann to use, he isn't as difficult as Prima, Tonio and Sonika.
Still as more information comes out, this opinion will likely change, but overall there hasn't been too many complaints.
Okay, here's a person who's used Oliver a lot. If you've worked with Japanese Vocaloids, or even harder to use 'loids like SeeU, he's REALLY, REALLY EASY to use. I was quite amazed to say the least when I first used him. He may need his BRI setting put to about 20-35 to make him sound clearer. But no, I haven't found any problems with him so far. ;)