We sailed on the Viking Legend from Budapest to Passau, and what an amazing trip! The crew were all friendly and went out of their way to make us feel at home; the guided tours were fun and yet I learned more than perhaps years of history lessons taught me about the areas.
If you've taken larger Caribbean cruises, set aside your expectations. This was a smaller ship, with less "amenities", yet more comfortable. While I didn't have a whirlpool, I also never spent time angling to get a seat somewhere.
The more intimate atmosphere lends to a more relaxed scene when you're on board, so in that sense, it's more relaxed, more low-key. However, the "excursions" definitely are more involved (and many are included!), giving more activity, and letting you see more of the real area, which I appreciated.