I was honoured to participate in the MemoryStoreService beta and I can say without a doubt that it’s one of the best services I’ve used so far. Excited to see the use cases developers come up with! I personally use it for matchmaking* right now to replace using permanent DataStores with private server keys and match data for the value.
It feels nice and intuitive, usable almost like regular DataStores. If anyone’s been waiting up for ephemeral DataStores, this is that feature. You can move some of your more temporary data over to MemoryStoreService, read it when you need it and get rid of it fast. Temporary persistence in general helps reduce a bunch of regular DataStore use cases I’d have originally and I’ve even been thinking of using MemoryStore to coordinate with DataStores.
Not looking forward to all those private server DataStores I’ll have to clear after DataStoreV2 is no longer an experimental setting though… 
*Just wanted to clarify for matchmaking since I got a few PMs on that: my use case hasn’t gotten to the level of cross-server matchmaking yet, sorry! By “matchmaking” I’m more referring to the definition of data for a private server than the complete matchmaking suite.