Over last three years I have passed a bunch of Esri technical certification exams just for fun. Some of my thoughts are here.
How to prepare?
Esri did a great job posting multiple blog posts regarding how to prepare for the certification: there is a free training seminar covering the details and some blog posts and Esri Australia blog post. Esri has also authored two special instructor-led courses for those who plan to become certified as ArcGIS Desktop Associate and ArcGIS Desktop Professional:
Another useful resources are free web courses:
I highly recommend going through sample questions to get an idea of what type of questions will you be present with on the exam. For those considering obtaining the ArcGIS Desktop Associate it might be beneficial to check the Esri ArcGIS Desktop Associate Certification Study Guide (Esri Press).
I believe that the best way to prepare for any exam is to work with the software in various ways according to your job responsibilities. You will know your stuff because you use the software daily, you might support your users or colleagues at work, and most important you will run into problems by operating the software and you will be in charge of fixing those problems either by reading through the documentation or talking someone at work or on the Internet. At the same time, it is quite common for many of us to be engaged only in some specific workflows most of the time, just as you said, so you might need to read the documentation or experimenting with the software first to make yourself comfortable at the exam.
If this is the case, your options are:
Go through the information page for the exam you plan to take. There you will find qualifications required, skills measured, and training resources. Try to go through as many Preparation Resources as possible to make sure you cover most of the topics.
Read the Esri Web Help. This may seem like a no-brainer, but it is really worth reading. There are some specific sections that are particularly interesting. Read through the “What’s new in ArcGIS 10.x” section to get an idea what features and tools were introduced in this target version (if you go to 10.2 exam - it's worth checking what's new in 10.2). But be careful reading the latest version of the Help because if you are taking a 10.1 exam, you don’t want to know anything that was implemented in 10.2. There is a link on the Esri Resources page to go to a previous version help which will take you to an earlier version. Don’t forget to go through the software system requirements and installation guides at the Esri Resources page as well.
Go through the ArcTutor (a set of tutorials with .mxd documents, GIS datasets, and tools) which is available for free for anyone who has obtained ArcGIS Desktop. Unfortunately, you can download only some parts of ArcTutor on ArcGIS.com; most of the tutorials are available only via Esri Customer Care Portal which you as an Esri customer should have access to. I found ArcTutor tutorials to be as good as Esri instructor courses skills-wise.
Esri Training should also be a main entry point for anyone learning about Esri GIS technology. There are dozens of free web courses with the access to the training data (.mxd map documents, geodatabases, and tools). The tutorials are updated regularly and include the latest software versions. Many users have no idea how good Esri web courses are; most of them are also available for free. However, avoid just going through the steps outlined; try to think first – why you need to do this and what will be the result. It might be good to try to do the exercise yourself first and then go through the steps outlined to see if you have chosen the right path.
Watch the Esri videos from user conference technical sessions and developer summits. This way you might be able to memorize things better since you will also listen to and not just only see or read.
Get yourself familiar with the software GUI because you want to know the name of the used tools, menus, buttons and panels. Usually people work with just some parts of the software, so no one expects you knowing every single button; however, it is worth playing with the software for some time before the exam to keep the GUI images in your memory.
Pay attention to the licensing levels of the tools and features. Again, no one expects you knowing in what license every GP tool is supported, but browsing through the functionality matrix for desktop is definitely worth it. You probably also want to know what extensions you need to run most used GP tools.
Get access to a physical or a virtual machine with the software of older versions installed if you don’t have it yet. Play around trying to access data stored in a newer format from an older client or move some piece of functionality into a newer version and find out if there is anything you have to do to make it work in a new environment. You will memorize such things much better by practicing rather than by reading the Help; so if you don’t work with some aspects of the workflows mentioned with the certification exam scope daily at your work, get to know them by getting your hands dirty back home in the evening instead of watching the telly ;).
Is it worth it?
I think so. It was fun learning some of the aspects of ArcGIS Desktop I did not use earlier and some of the skills I've got while preparing for the exam I actually find use for in my daily workflows. I have never heard (and neither used) of Data Reviewer extension before, for instance, but use it a lot nowadays. So, just learning what other parts of ArcGIS are out there can be actually very helpful since you will be able to make more awared decisions regarding what tool or technique should you use in various situations.
I am urged that this is a kind of investment in your knowledge and ArcGIS platform expertise. If it will be hard for one to get to the Esri courses, there are lots of free courses available and you can definitely pass the exam without taking the class.
I've also noticed that for some job ads, as the Required: (or Preferred:) it was ArcGIS Desktop certification which means the market is aware of this certification and it might be a good thing to have on your CV or profile, imho.