I've been wondering to a while now if I should get this game because it's really not all that great.  However it's on sale today on steam for $20.  Is it even that significant in the storyline?

For $20 why not? I got mine for $30 and I haven't even opened it yet lol. As for importance to story, I have no idea but I'd say it has to have at least some importance. Unlikely that it is a complete filler.

For $20 I would get the game. I haven' beaten the game myself yet. Imo it doesn't have as good of a single player as AC2 or Brotherhood, but still decent. Plus the multiplayer is really fun.

Sure...it's not that good but still a bargain

Just beat it today its pretty decent. Brotherhood is still the best game in the series in my opinion.

I love Assassin's Creed. They are all far too easy, at least from ACII and onwards, and too accessible like interactive movies but I simply feel so great when playing Assassin's Creed because of the atmosphere and almost dreamlike flow of the game. Plus they are big games.

I'm currently playing Brotherhood which is a bit overdone with all its meaningless features but I still want to experience the complete game, I play all side missions, all DLC and I will do the same with Revelations.

I don't quite understand why everyone keeps saying it is not a good game. The tower defense and bomb crafting is not really used that much in the game. In fact i didn't have to deal with any tower defense at all since i didn't let the wanted meter go up. The frame rate was bad at times on the ps3 version i played. Didn't try the multiplayer at all so get it if you like the gameplay, the lore of seeing an ancient city come alive and to finish the story of Ezio.

Cheers.

Its good, bt doesn't really compare to 2 and brotherhood its definitely worth 20 dollars though.

20 is too much

For 20? Sure, worth it.
For anything above 20? No.