I use it all the time. It is great for singles if you are a PCing a player or looking to fill in sets as they start from 0.18c rather than the higher starting points on Ebay/COMC. You wont find a lot of higher end cards in the listings though. They do come up in the auctions sometimes.
I just get orders sent to the box then ship that when I feel like it. There are weight break points for international shipping so I try and maximise the number of cards right up to a break point before shipping the box. No point being an once or so over but having to pay and extra 10 or 15 USD to ship. You can choose to only ship part of your box if you want to avoid this.
They do use USPS standard shipping and have no other options so it is not the fastest. I am going to get my next shipment sent to Shipmycards first then try that way so I can use Fedex or UPS as this is faster (and I feel safer).
Any other questions let me know.