I'm not the biggest fan of Assassin's creed in the world, and I'm the last person who will say it's perfect. It is, however, a fun game, and one that did things that other games didn't, and nowadays, it's probably on sale, so its short length is mitigated by the reduced cost, so if you haven't played it yet, I'd advise picking it up.
I'm mainly writing this to defend a particular part of the game: the repetitive missions that everyone complains about. I think people are complaining about them because they're not looking at them with the right mindset. First of all, if you don't like sitting on a bench and listening to people, I think the game only makes you do it once. Ever. Think of it as a brief cutscene if you absolutely can't stand it. Second, it all works if you think of it in the context of planning an assassination. You're not doing the side missions because you love to listen to people on benches; you're there because information is critical to an assassin. It's true the game doesn't sell it very well, but it's all awesome if you think of each chapter as laboring to produce the perfect assassination. If you do all the little labours, you can find out where the target will be, what his deal is, and the safest route. Getting the perfect run is tricky, but it's very rewarding.
Now, other complaints are completely legitimate: the game is far too short, with only 9 assassinations, and the first one is comically easy (still cool though, made me feel like a bad-ass to invisibly assassinate the guy, then walk away with no suspicion on me, on the first try). The cutscenes are very poorly thought out, especially having to watch the deathbed confessions of each of the targets. I've said it before and I'll say it again, cutscenes that are absolutely unskippable, even on a second run through, are a crime against humanity. Nothing kills replay value faster.