For the most part we have found that using ISBNs with the CCM tool has produced excellent results. However, a couple of things about the way ISBNs are used in cataloguing and how Copac records work mean that searching and de-duplicating by ISBNs in CCM can occasionally be dangerous.
First, ISBNs for series are often recorded in the catalogue record for individual books in a series. So, for example, a search on CCM for ISBN 9025606199 (for the series Byzantinische Forschungen) returns 30 Copac records. Some of these records are for the series, but there are also records for 14 separate titles within this series. With ISBN de-duplication this is reduced to just one record. This could give the impression that a book that is in fact unique to your library is held at multiple places. In our opinion, therefore, unless you can be sure your list contains no series ISBNs, ISBN de-duplication should not be used for decisions such as withdrawals.
Secondly, it is common practice in cataloguing to record a print ISBN in an e-book record. Sometimes the 776 field is used, or the $z of the 020, but there are also a lot of e-book records in which the print ISBN is included in the 020$a (sometimes, indeed, the ISBN is shared by print and e-versions). But in cataloguing there are distinct records for the print version and e-version of a book, whereas this does not seem to be the case in Copac records. So, for example, a search for ISBN 9780230238978 returns just one record, even though the holdings for it are both print (at e.g. UCL) and electronic (at KCL). This could give the impression that a book that is in fact unique to your library in the print version is held elsewhere, whereas all the other holdings are for electronic versions. There is detailed information about the holdings in the full record, but for library staff to check these by batch would be very difficult, if not impossible. For withdrawal decisions the apparent merging of print and electronic resources on one record is problematic.