Hanumant Deshmukh Ii wrote: You can always learn new Java features without appearing for the certification exam.
I can only talk about my experience here, but I found that, unless I pro-actively start using the new feature right away (not always possible at my current job), I will forget about it almost instantly.
One advantage to take the exam is that it forces you to study the new feature
really well, instead of brushing through it quickly. Which may or may not help you remember it better in the long run.
Up to one's own judgement if that, on its own, makes the exam worth it or not ^^