I had the opportunity to play this class in the Ravenloft setting of 5e Curse of Strahd - and I have to say that it works perfectly in Barovia. The combination of supportive capabilities, spooky vibes, and the ability to conceal a member of your party really worked well to support the dark action and intrigue of the campaign. I also found that building a Spirit Guide made it surprisingly easy to integrate my character into an already existing party. You have the capability to take any npc that the group has recently seen perish (or any already ghostly npc) and convert them quickly and easily into a Spirit Guide player character. This worked particularly well with the haunted introduction to Curse of Strahd.
If you are a DM, I would recommend picking this class up to offer to your players - to give them an extra dimension to explore when creating new characters. Whenever a player character dies, but wants to keep their character on, you can now offer them the chance to continue with whatever drove them. It also provides you with a creative way to introduce new PCs if you have a party that people are always joining.
If you are a player, then you should feel comfortable offering this option to your DM when building your character. The Spirit Guide is well-balanced and specifically designed to avoid changing the rules too much - or breaking existing expectations. Other than thematic choices and the possession mechanic, these sub-classes have been built from the ground up to feel just like any other supporting Cleric, Bard, or Warlock. If you are looking for something interesting to try out, then there is a good chance that your DM will work with you to integrate your new Spirit Guide into your campaign.
At the end of the day, I loved this class - and I can't wait to get the chance to play it again.