There's still a learning curve even with a controller, but once past that the controls have a beautiful simplicity and responsiveness that I've seen in very few other games.
The "everything on one account" approach is really, really nice. All my save information is transparently synced across devices, and I can buy something from my computer (or even from my phone) and play it in my living room without any hassle.
That is the beauty of combat in Dark Souls: it's not mindless button mashing, and it's not about precision aiming. It's about timing, reading your opponent, thinking ahead, and getting the right rhythm - it's more like dancing than combat, really. On top of that each weapon feels different to the point where you choose to use the weapon that feels right for you, rather than based on stats. It's a bit difficult to explain why, but that connection is a lot harder to achieve on keyboard+mouse.
But... It does come with a giant red warning... From the Steam page itself:
Notice: Microsoft Xbox 360® Controller for Windows® (or equivalent) is strongly recommended.