I think it depends on your goals for obtaining certifications. Certification books, guides and commercial practice tests for popular certs like Cisco, Oracle 11g, and Microsoft are plentiful and they will prepare you to pass the exam. If you need certs for continuing education credits, promotion, this is a fine way to go. You'll cover the topics required for certification and that's all. The problem with all certifications is that these topics are selected on what the company or publisher thinks are important to learn, which at times may not reflect skills needed by an employer.
If you're interested in going beyond the certification requirements, and I highly suggest that you do, research the requirements and topics for a given certification and develop your own study guide. If possible, download the software, or find simulators. Take the requirements and test your answers on a development system. You can use Virtualbox to create an entire "lab in a box" to understand why the answer is correct. Read the manuals, break things then fix them. If your current employer uses the same technology you want to get your certification in and you work with it every day, that'll help tremendously. This approach will help you pass the exam and interview well.
Personally, having certifications has helped me get interviews, but experience is what got me the job.