They're great fun! Take a week or two off work and use it to explore, chill and enjoy riding your scooter through some great roads and scenery, sampling local culinary delights along the way, not forgetting the people that you meet all add to the mix.
Will you be riding alone or with mates? Don't be scared of going solo, particularly if you're handy with the spanners, make some contacts and use the forum or Facebook etc for support or assistance along the way. I've found riding solo or with one or two others makes it easier to interact with locals/people along the way if that's your thing.
The Euros mean different things to different people but I would guess that pretty much everyone enjoys the camaraderie and the excuse to have a drink or two with like-minded others that have made the trip. The rally is the reason to go but it's only the half-way point of the trip.
If your going to go directly to the rally next year then I'm sure you'll enjoy it but it is only a day's ride from the ferry or tunnel terminal. If you've got the time go exploring, me and a mate are heading to Gent then the Black Forest and the B500 Schwarzwaldhochstrasse. The German roads and scenery are particularly good.
Whatever you do do it, you won't regret it
Mark