I have a lot of experience with Petra. I've sold about 40 items from Petra since I started my eBay store in April, and they are most definitely THE BEST Drop Shipper I use. While their prices might not be as low as other suppliers (leading to lower margins per sale), they make up for it with the product quality and customer service.
Every order I have placed with Petra has been totally hassle free. I have a dedicated Sales Rep who is actually a REAL PERSON and was been VERY HELPFUL answering all my questions. She typically replies to any e-mails I send her within minutes. Their shipping is also very fast, sometimes shipping out the same day I place the order or worst case next day. Their inventory levels are very accurate, so you don't have to worry about something being out of stock when you sell it. The products are all brand new and excellent quality. I cannot think of 1 negative thing to say about Petra, but please feel free to ask me any specific questions you may have.