If Saint's Row 2 is your favourite game ever, then I think the first game will be worth playing.
Despite being an 'inferior' version, I think you'll still find it worthwhile for the back-story and characters. I think after playing it you'll appreciate how Stillwater has changed and exactly what Ultor had done in the time between the 2 games.
I believe that the Saint's Row series was written as a trilogy, even before work started on the first game, so the original is still very relevant to the story.
There were also a few features and vehicles that were unfortunately omitted in Saint's Row 2, so you'd enjoy playing it and discovering those.
The characters like Julius, Gat, Troy, Lin and Dex are enjoyable to meet in the fist game, plus the other gangs have interesting characters, like Benjamin King and 'David Carradine'.
I think SR1 has enough acting talent to make it worth your while to play through the story missions at least, but you may not want to 100% it.
Last and but not least, the radio stations are a lot better in the first game, as well as having a lot better selection of hip-hop and classical, the right-wing talk radio station is hilarious.
You can probably buy it quite cheaply now, and for a fan of the series it will be worth the little money it will cost, plus it's not a huge step backwards, technology wise, for example it wouldn't be as retro as playing GTA III after GTAIV.
