Estonia was the forth country during my first trip. It's in the EU, and things work pretty much like everywhere else.
They use the Euro, ATMs are plenty, and you can pay with card pretty much everywhere. Mobile networks work the way they should, and you can use EU roaming here meaning any EU-based SIM card works just like at home (with some conditions)
Buses go around the country, and there are some trains also. I used Ecolines, which is the local bus operator, and Lux Express, both were rather unsurprising, albeit the drivers were a bit too reckless. Otherwise, you can buy your ticket online and board the bus with a printed ticket.
I thought about going from Tartu to Riga on a train, but that would have meant a connection at Valga, so I took a direct early bus. So I don't have any experience with trains in the country.
One surprise was the VAT, which is usually included in the price elsewhere, but not here, at least not for services that is usually bought as a company. This made the coworking space rather expensive. But don't worry, restaurants don't charge for VAT separately.
As I had only a little time, I did not travel around in the countryside, but my first impression was that it has a lot of forests. Most of the bus ride was next to one, and everything seemed green.
As for costs, it is definitely more expensive than Lithuania and Latvia.
Tallinn was my first destination in Estonia, and going here was the primary goal for my first trip.
After Tallinn, I had to make a choice. Either take the ferry and visit Helsinki, or take a bus and go to Tartu. I chose the latter and I'm glad I did.