The main reason they are so underrated is based on because they are in the AFC South,which by far is the hardest division to play in.
The Jaguars went and got Jerry Porter from free agency this off season.They also traded Marcus Stroud to the Bills for a couple of 2008 draft picks.I believe they had a really good off-season so far this year.
Coach Jack Del-Rio put his job on the line when he got rid of Byron Leftwich,and put all his trust in David Garrard.No one that that these same Jaguars would of not only make the playoffs but also be the only team in NFL history to beat the Steelers' in Pittsburgh twice in the same season(week 15,and then again in the wild card game).
They also have one of the best two-headed running duo in the league.Fred Taylor and Maurice Jones-Drew pretty much ran over the defense when they were on the field.The Jacksonville Jaguars were 2nd in rushing yards/game in 2007,just close behind the Minnesota Vikings.
As long as they can keep pace with their long time rivals(Indianapolis Colts),they should be back to the same Cardiac Cats like they were back when they fist became a franchise.
My prediction for them this year is that they will win their division and finally make the Super Bowl,and they will get their first championship ring in team history.