Clarence I've done a fair bit of research on this since we're looking to expand into a chain of hostels.
We had short-listed ASSD and Charts but my choice is ASSD with he main reason is the interface capability. For me saving labor on manually inputting reservations and updating inventory/pricing is very important.
ASSD is the only PMS with a "true" interface to hostelworld.com where there is an agreement in place. Hostelworld has confirmed to me that this is the only PMS that has an authorized interface to them.
Maybe chances are small that Hostelworld will disable the interfaces with the other PMS but it's not a risk I can take bearing in mind the investment.
Having said that we haven't purchased it yet and I haven't spoken to many people who are using it so please bear that in mind when readying my comments.