Prior to watching The Unnamable I had seen Two other Lovecraft Masterpieces (From Beyond, Re-Animator), while The Unnamable doesn't get under your skin as much as the other two it's well worth a look.
The setting is perfect as it takes place in an old big house which has supposedly been haunted for years by the most terrifying creature. The creature is so terrifying it doesn't even have a name, hence, The Unnamable. A few college students, from nearby Miskatonic University decide to go and explore the big, creepy house. They soon will find out that would be the biggest mistake of their lives. As with all Lovecraftian films there is some nice gorey scenes and some startling moments. Also The Unnamable is one of the most frightening creatures you'll ever see.
I feel, however, The Unnamable is lacking compared to the other Lovecraft movies I've seen (mentioned up above). The acting in The Unnamable is really pretty poor, the picture is very dark and is hard to see exactly what is going on in some places (I only saw the VHS version) , and we only get to see the creature for about the last 10-15 minutes of the movie. All and all The Unnamable is a good movie and if you're a fan of Lovecraft films I don't think you'll be disappointed.