I've used shipwire for 2 years now and have had constant problems with them. Currently, we have packages that have not been set out for several days and no reasons why. They do not have efficient contact with their warehouses and my customers have received random products from other companies storing at shipwire 3 times already. Their online system is easy to use but the service provided and problems that occur are a greater hassle to fix than the ease of the online system.
There have been a lot of growing pains with Shipwire over the last 2 years.
The interface, fulfillment are great and works seamlessly with Shopify - but when a problem occurs expect to work through a problem with them over several days or weeks. There are still logistical issues like mixed orders and missing return packages and the ease and pleasantry to resolve these are still lacking. In many instances the growth of the company feels like it has pushed aside small vendors in favor of larger ones - this is true in the policy changes for minimum orders and the once personal phone support to now having ongoing return calls and emails as followups to work with higher call volumes but less efficiency.
The platform and user interface is well developed, however customer service is very slow and often unhelpful. Fulfillment is a mission critical component of any e-commerce operation and any issues need to be resolved asap, and for them to take days to even respond to support requests can really negatively impact your business. If you're a relatively small shop, Shipwire does not care about you.