I ****ing loved the first one.
I don't know if I can ever really go back to it, because honestly, and speaking as someone who seriously loved the first enough to 100% the achievements for it, the series just made massive leaps forward with the second.
If Ezio hadn't come around, and ACII been as good as it was...that series easily could have just died then and there. There had been a ton of hype with the first, and mystery. And people seemed letdown by in large, but enough were convinced of it's potential for it to get another go around.
If you've never played any of them, I'd recommend playing the first. It's always good to start at the first if you can. The game has it's problems, repetitiveness seems to be the label it always got, but it's functional and not broken any horrible ways. Slush through it, and I think you can gain a certain appreciation for the leaps made in the second. I also recommend it because it could just be too hard to start with the second and go back to the first without that one feeling...empty. Less to do, etc.
That, and you'll see a different approach to the storytelling. I mean, compared to the first one, you kill a TON more dudes. Like...mass-****ing-murder number of assassination compared to the first one.