My current HO team uses Gengar as a spinblocker, because as you said, he's one of the few spinblockers who can maintain offensive momentum. My only problem with him so far is that he loses almost 100% of the time to Starmie, who outspeeds any non-Choice Scarf Gengar and OHKO's with Psychic. Gengar also has problems with Tentacruel, who has the SpD to tank Shadow Balls for a good while, and can hit Gengar with both Scald and Ice Beam, meaning he won't be forced out by Disable.
All that aside, Gengar is a monster and a godsend for most offensive teams. 'Dat base 110 speed means he can outspeed the plethora of base 108's crowding OU, and his combination of Ghost-typing and Levitate means he has plenty of opportunities to switch in. My favorite Gengar set so far has been his Sub/Disable set, as he can force switches on a lot of Pokemon who only have one move to hit Gengar with, such as Terrakion or certain Rapid Spinners, forcing them to eat entry hazards while he either sets up a brand new Sub or unleashes a mighty Shadow Ball/Focus Blast.