I started using the add-in 6 months ago after using GTD with Outlook without it for over 4 years. I regret not purchasing it earlier.
It comes into it own for linkimg emails to actions/filing emails/defering email actions to future dates. Also grouping actions by project is good if most of your actions are @office based in which case grouping by context isn't as useful for you as it is for (say)David Allen who spends a lot of time on travelling!
I love it and recommend it if you get loads of emails that you need to process fast and if grouping by project really helps you.