As a main unholy player I have to say that the main issue with this spec is gear scaling.
When I was 340 I was destroying the meters, now me and the rest of my raid group are 350+ and I'm down to 4th/5th in a list of 10 dps's.
Apart from this, it has two issues: lack of burst (even with gargoyle) and all the ramp up you need to start doing some serious damage. This latter issue also makes up for a terrible cleave, change target, you have to ramp up everything again.
The only good thing right now is AoE, it's a monster!
I'm only waiting to get a second decent 1h to go back to frost, love unholy but it's too frustrating right now.