Personally I don't regret having bought EU5 at all. Even the 85 Euro I paid for the premium edition means I only paid 50 cents per hour up to now, and I am far from finished with this. I can see myself playing this quite a lot more in the coming months, and when I eventually want to move on, I can see myself coming back repeatedly over the coming years.

Having said that, EU5 is an extremely slow game. On my computer a year on the fastest speed without any interruptions takes nearly 2 minutes, or 3 hours for a century and 15 hours for the complete five centuries. If you actually play, a full game takes at least 50 hours, and if you like looking and managing many details, it can easily be 100+ hours for a single run. A full game of a typical 4X strategy game would be a lot faster, so EU5 already isn't for everybody because of that.

My main purchase warning is related to this: While EU5 is perfectly playable as it is, the developers are currently very busy with patches, and those patches can massively change how whatever country you chose plays. For example, this week patch 1.0.8 moved from beta to live, and it messed in a major way with the loyalty of your vassals and the centralization / decentralization values. I was lucky that I started my game on the beta version, so now I just switched to the live version without any changes affecting me. But some people's ongoing games were seriously messed up by the changes.

If you combine frequent patches that introduce massive changes with a game that takes up to a 100 hours to complete, and people who might not like me have 40 hours per week to play, the risk of your game being messed up by a patch becomes rather significant. If that is something you dislike, and you don't want to play a game that feels a lot like early access with regards to devs still experimenting with major game mechanics, I wouldn't recommend buying Europa Universalis V. If you don't mind the changes and the potential chaos, then there is a lot of fun to be had here, and I recommend the game.

Please note that these massive and frequent changes also affect any content you watch on YouTube. If you see for example a video telling you that you absolutely must strive to maximize your centralization value, regardless of what country you play, that video was only correct until patch 1.0.7. Under the current 1.0.8 version the advice would be a bit different, depending on your number of subjects.