It is indeed unique, there is nothing else you can really compare it to. Even the sequel seemed to have missed what it was that made the original work so well.
That said, and judging by some of the reviews I've read, players who haven't played it previously have not been impressed, whereas those that remember it fondly love the HD release anew.
So it's possible it will polarise opinion for the new generation and only appeal to the old. Will be interesting to see what the uninitiated make of it.
Regardless, it's a day one purchase from me. I've already stocked up on MSP in preparation. Only hours to go now