I have a hard time not believing the haters aren't disingenuous nay-sayers, since this was an absurd partisan issue that was thoroughly aired almost a year ago.  But for those who genuinely wonder:
1) Sole-sourcing.  Sounds like there was no competitive bid process.  But in fact there was.  It happened in the Liberals era.  Boeing and LHM were the only manufacturers that presented a bid with a working proto-type.  If your really want to dig, you would discover this is actually sourced from numerous companies all over NATO including Canada, who's aerospace industry has already earned hundreds of millions sourcing to the project.  The bidding process continues as hundreds of companies are constantly competing and bidding to provide the parts that make this plane.
2) Build our own.  Most Canadians are nostalgic for the Avro Arrow we designed in the 1960s for good reason.  But in this case, designing and building a 5th gen plane costs 300 Billion dollars.  Is it reasonable for Canada to take on a project that will add 300 Billion to our debt (remember Canada's total debt from decades is around 600 Billion. )  Considering the hew and cry of spending up to 30 billion for aircraft now.
3) Alternative planes.  Unless you buy off the Russians, there are no alternatives.  We have the choice to build superior planes, far above the playing field of today's fighters.  In World War II Poland saw no need to build tanks when their horses were just fine.  And they were fine--the Polish cavalry was one of the best.  The current version of the F-18 hornet suggested by some to be the way to go, has a kill ratio against the F-22 and F35 of 0 (yes that's zero) to 100.  The enemy in 5th gen planes could kill 100 Hornets without losing  plane!  When it comes to warfare technology, you don't want to field equipment that is as good as the enemy, you want to field the superior equipment.  It has been the vast superiority that has decided every war we have fought since WW I.
4)Buying from an American country.  Yes the main assembly will be done at LHM in the US.  BUT the equipment is going to be built by all participating NATO countries.  Canadian aero space has already made hundreds of millions of dollars providing Canadian equipment for the project.  LHM expects to spend 12 Billion dollars in Canada in comparison to the 9 billion quote.  Even supposing the price balloons to 30 billion; if the Canadian factories have zero inflation, we would be buying for at least 30% less than anyone else.  AND will create thousands of high-paying jobs in Canada.

Finally one important point.  We are not buying these planes for 2012.  We are buying them for 2030.  It is so easy to be backwards looking to save a buck.  But this direction is a direction that has a firm grip on the future.