According to the GIAC website:
GIAC exams are open-book/open-notes, therefore you are allowed to bring any non-electronic resources you may need during your test (this includes a notepad for writing). You are allowed to bring as many of the chosen materials as you are able to fit in a standard-size bookbag (roughly).
http://www.giac.org/exams/preparation/allowed-materials
For the CISSP, page 11 of the registration form states that:
Reference materials, except as indicated below, are not allowed in the testing room.
The only exception is this:
...word-to-word language translation dictionaries are permitted
for the English examination, should you choose to bring one to assist you with language circumstances.
https://www.isc2.org/uploadedFiles/Downloads/exam_reg_form.pdf
It may be worth adding that most certification exams do not generally permit examinees to bring their own resources to the test. I personally have sat for CompTIA, Microsoft, and Cisco examinations and am looking at taking an ISC^2 test in the near-ish future - none of these allow the test-taker to take anything more into the testing area than whatever the proctors issue. Typically, the only things issued are a marker and whiteboard, and perhaps a simple calculator. Sometimes, a pencil/pen and paper are issued instead, but the paper is shredded after the exam.
Though this is wandering into the realm of subjectivity, I personally would wonder if a certification that allows open-book testing is truly regarded (by the industry at large, hiring managers, etc.) as having "equal status" to those that require candidates to rely only on their personal knowledge.
According to some colleagues of mine, and others here in comments, it seems that the GIAC exams do hold a good bit of weight on their own. The Silver exams alone are difficult enough that having open-book resources on hand is a practical necessity to pass. However, at the same time, the examination period is short enough that one cannot solely rely on the books for all the answers and still finish in time. Additionally, the Gold-level exams require you to write an original paper, better demonstrating a deep understanding of the material.
