Dark Souls Remastered was supposed to arrive on May 25th for PC, Xbox One and PS4, but for some reason the PC version is now already live and available for purchase. From what I've played so far it doesn't appear to be any sort of early build, so if you were itching to give the Remastered edition a try, feel free to jump in at any time.
In terms of improvements, the Remastered edition has brought in slightly better visuals, significantly improved performance across the board, 60 FPS even in the depths of Blighttown, better online connectivity, as well as a small visual overhaul of fog gates and bonfires. It's nothing too spectacular, but if you're curious about what the 'new' Dark Souls looks like in action, allow me to share with you the recently posted launch trailer. Have a gander, it's quite an exciting one:
Since the old PC version can achieve most of these things through the use of mods, owners of Dark Souls: Prepare to Die Edition on Steam will be getting a 50% discount towards the purchase of Dark Souls Remastered. Whether it's actually worth the asking price, that's a bit of a tricky question to answer.
I'll post a full review once I complete my playthrough, but so far I have to say that the Remastered edition is more like a beefy patch than a full remaster. It makes the gameplay a lot smoother, and I especially appreciate the consistent 60 FPS, but there has been no effort to actually improve on some of Dark Souls' shortcomings or even fix some of the most glaring bugs. So if you already own Dark Souls and you're wondering whether it's worth spending €20 on the Remastered edition, I would say it depends entirely on how much you desire a smoother online experience since nothing else has really changed.
If you're a completely new player, however, I would fully recommend you give Dark Souls a try since it's by far my favorite game of all time. You can learn more about it, as well as grab a copy for yourself, by heading over to Steam. Have fun!