I actually played Sacred before I got around to playing Diablo II, and I still like it a bit better. The combat etc. has the same clicky-clicky feel, so fans of Diablo will feel right at home. A few of the highlights:
*Crafting/modding equipment: it works, it's easy, and it's needed, especially if you want to survive on the high difficulty settings. I hate how crafting items is rarely useful in Dungeon Siege II and other games- if you can't make an item any better than what you have already just *found*, what's the point?
*Soundtrack: it's been a while since I played, but it was great.
*Open World: you feel a lot less constrained than in Diablo, and having a world that isn't randomly generated has big upsides in terms of giving the game a more solid feel, while still being HUGE and intimidating and exciting to explore.
*Character Classes: there are several, and they are very cool and different.
One downside... Sacred only runs at 1024x768 max. At that resolution, it looks gorgeous, but scaled up on an LCD... not so much.
I'm seriously considering buying a digital version here from GoG for my netbook. I own the CDs, but having a version not requiring a CD for install/play would be great..
Also, I would say Sacred 1 is better than 2, if only because you don't have an annoying 3D camera to deal with all the time...!