I played it in coop, from beginning to somewhere near the end.
I liked it, but i don't think it can really compete with Diablo II. It's more a rival to Titan Quest oder Silverfall, both are "nice"; but not "great" or even "awesome".
I don't think the character classes had something unique. Sure, you do not see a seraphim that often, but it's not like it had any influence. If you get some glowing wings or so, if there are, you may notice a difference, otherwise, it's just a spellcaster/melee combination, depending on what you prefer.
And Vampire.. every necro has some life leech skills and so. Nothing special, too.
As i said, i played it in coop, with a friend, and our chars where almost never on par, which takes away from the fun.
It's not like enemies would have been a problem, most times. Especially bosses where way to weak. Some giant dragon or spider, and a few seconds later he is dead. Hordes of orcs or stuff where far more annoying.
The free roaming was, well, it was a nice idea, it allowed for many quests, but also you had no line you could follow and so. Most quests where simple, retrieve this or that or kill 10 orcs and so.
It is not the amount of quests that counts, it's wether the quests are real quests ,and not something your neighbour could do, if it had any "impact" on the world/story. And with dozens of quests, you just can't do that.
Also the skill system is different from most games. And with that many possibilities, it is most likely that you will skill wrong.
Also the map was that big, because they did not make "levels", but a real map. But that leads to much not release used space. Wether i'm in a wood in the southeast or in the middle north makes no difference.
Also there where some annoying mountains with only one entrance. You had to do a lot of stuff nearby, but you had to go all around if you wanted to get to the other side.
It is fun, but i think they wanted to achieve to much. They wanted a different skill system, different characters, more skills, more quests, more areas to explore..
There is only so much you can do without repeating yourself.
Diablo I and II had several different areas, all completely different from the one before. You just don't notice the area really in Sacred.
It is fun, especially in coop, but it's not as polished as Diablo II. If you like hack & slash like in Diablo II, and free roaming like in Morrowind, it's nice. There is also a demo for both Sacred and Sacred 2 if you want to try it before you spend money, even if it's not that much around here.