Well, since this is kind of the "Don't use classes in your CLA exam solution" thread, I figured I'd chime in with my $0.02 CDN now that I've passed...
I spent a long time thinking about whether I'd use classes going in to the exam. I received strong advice from a few people not to, but also bought into Steven's statements above about his experience. In the end, I decided to go with classes for 2 reasons: Firstly, because I'm now way more comfortable using them than not using them (and I hate the sample exam solution's use of "action engine"-type VIs, with tons of non-required inputs), and secondly on principle - Architects should use classes, and if the examination mechanism can't handle that then it needs to be challenged.
My use of classes was quite straightforward, and seemed obvious (to me at least). All of my VIs and controls belonged to a class except my top level VI. The only issues the markers took with my architecture were to note the areas where it wasn't complete or properly documented, which was completely fair (since it wasn't - I ran out of time). I'd say my biggest mistake was getting sucked into too much "implementation", at the expense of completing the architecture. I am certainly grateful for this thread though, since it raised the level of discourse about this issue before I wrote my exam, and Zaki spent a lot of time addressing this in the exam prep session I attended.
I would say, however, that using classes probably takes more time. There's a lot of "background" work to creating a good class hierarchy, with icons, documentation, data member access VIs, access scope, etc. If you're confident in your ability to create a non-OOP solution, you might consider this. Time was a huge factor for me, and four hours of hard-core, non-stop LabVIEWery is exhausting.
And here's a tip I struck upon the night before my exam: Create a quick-drop plugin to prompt you for a requirement ID and automatically create a "[Covers: XX]" free label. This only took me about a minute (I memorized the steps before the exam), and probably saved me 20. Make sure you turn on scripting in Tools-Options, then copy "C:\Program Files\National Instruments\LabVIEW 2010\resource\dialog\QuickDrop\QuickDrop Plugin Template.vi" into the "plugins" folder, rename it, and edit it to create a Text decoration with the appropriate text. You can even use the "Prompt User for Input" Express VI . Then assign an unused key (hit Ctrl-Space, click on "Shortcuts...", then the "Ctrl-Key Shortcuts" tab, and your VI should show up in the list). For me, adding a "Covers:" free label was only a few key-clicks.
Jaegen