I don't know how I first heard about Jericho. It definitely was not through normal advertising channels (which says quite about CBS's anemic promotion). I think it was a combination of io9 coverage with netfix recommendations. In any regard, I'm so glad I discovered the show and now I have a chance to support it before CBS tries to cancel it for a second time. I discovered Firefly too late (a year or so after Fox canceled it... actually same with Family Guy). These studios have to realize they have to give these shows a chance. I've put my money where my mouth is and purchased season 1 and I urge you to buy or rent it as well.
The great thing about this show is that the plot does not seem absurd to me. Whether its 9/11, Katrina or this administration, but in the late 90's, this would have seemed more fantastical that it does now. They do a good job of walking a fine line between what is constructed to make it a good show and what might actually happen.
And to boot, the cast is fantastic. They really have pulled in quality actors to sell this, what is ultimately a story about the human condition. Combining that with a great who-dun-it mystery (which I really hope they have an opportunity to reveal with good pacing), you have the makings of quality must-watch TV.
Now, to answer the question is it worth getting invested in the show (especially one like this) when it might not have a season 3 after only a 7 episode season 2? I think the answer is yes. Season 1 is a full 22 episodes and while it doesn't nearly wrap up the mystery, Season 1 + 2 give you enough of a satisfying plateau that it's worth watching even if there is no season 3 (keeping my fingers crossed though).
Hopefully the executives won't make the same mistake twice...