Yes, this game ain't have excelling graphics. It has some archaic/weird solutions. Yes, in terms of gameplay it can be called a clone of Diablo. Is it mediocre then?
Definitely not.
What's different from other Diablo clones is the fact, that this game has quite an interesting setting, witty dialogues and is not another pure hack n' slashing thingy, as it has really well written story, and it's creators had some original ideas (such as attunements, causing you to try almost every piece of equipment for some time to receive different set of modificators).
I recommend giving it a try, and beat it at least one time, because it's worth it - beginning may seem a bit harsh, but the more further you go, the more interesting it becomes. It's a well done mix of action RPG with something close to classic, plot focused RPG's, a thing not likely to find in gaming nowadays (none shall prove me otherwise).
I don't give it the fifth star, because I'm aware that most of my impressions about this game were found about decade ago, when I was younger, and I feel strange nostalgy for this title, and I'm aware it's not perfect. But for someone who likes atmosphere and nice plot and well written dialogues, this game might be a good choice.
PS. GOG, you did something incredible - you made this game running on Windows newer than XP. I'm impressed, as nobody so far managed to do so (I have non-digital, DVD version of this game myself, and can impose my computer to run it)...