I thought the same as you, but decided to take the plunge and buy the game and must say I'm glad I did yes it is short and yes slightly repetative(but I also thought bioshock was also short and repetative, a all to common trend!) but loved the experience (haven't tried multi player).I would say rent if you can but I did not regret buying it!
I've only played the demo but find it very reminiscent in gameplay as games like Max Payne and Dead to Rights. I wasn't too impressed with them but from the Stranglehold demo I did find that the controls were well implemented and things like the slow motion diving and gunplay appear to be executed to a superior standard to previous games.
However I can't see it being anything more than a quick fix and a game of this kind being longer would surely dull it's appeal and highlight the repetitive nature of the action. I think I may save this for when it becomes a bargain basement item as Bioshock is vying for my time (and Halo 3 vying for my money)
I bought the game on the basis of the demo being amazing, however it is disappointingly short... Bioshock is much longer by comparison. Though Stranglehold is repetitive, it does however have some replay value... harder difficulties, achievements etc. I haven't delved into the multiplayer much, but it seems moderate. To be honest I was a little disappointed with the game: don't get me wrong, its definitely a blast, it just could've been more I think. The main issue is definitely the length though.