I used it every day for a couple months earlier this year. There are lots of little differences. I thought it was a more interesting place to be yourself than it was to find interesting accounts to follow.
Twitter is really good at the latter. Especially if you are looking for people to THREAD you up some knowledge or if you like viral content.
But there's also that saying that goes something like: "Every day one person is picked to bear the shame of all of Twitter, and your goal is to not be selected as that person" --or something like that. It's not exactly a chill microblogging platform anymore, but Mastodon is.
So, while Twitter might be a good research or learning tool, and Mastodon could also be that _depending_, Mastodon is also kind of open to exploration without some of the pressures or distractions inherent in the Twitter model.
If you want to microblog in a chill way and build up some fun toots to look back on over time, it's probably a fantastic platform for that.
Personally I took a bit of a Mastodon hiatus to explore SL and OpenSimulator, both of which are also pretty interesting.
http://opensimulator.org/wiki/Main_Page
https://www.youtube.com/watch?v=2E4t8y9k1Us (Video showing some interesting worldbuilding in OpenSimulator)