how essential are pro flowers?

my #1 goal for my wedding is to have fun.  so i'm torn on day-of or day-before tasks that could detract from that.

professional flowers are just absurdly expensive.  can i really justify spending more on flowers than on renting the entire venue?!  this place:  http://www.bloomsbythebox.com/ seems really neat.  order your flowers, they arrive that day (or the day before), put together your own bouquet and centerpieces and BAM.  you just saved like a grand.  maybe.  i have no idea how much it would save actually.

i know my bms would be happy to help with that stuff.  but i'd have to make sure we had the same ideas on how they should look.  and, i want THEM to have lots of stress-free fun at my wedding too.

a lot of weddings have a cool diy/natural vibe with lots of little 'off' details that all just sort of work, so that 'imperfect' flowers look just right.  but that isn't the look i'm going for with my wedding.  i'm definitely going for more of an 'elegant' look...with some quirky details.

would arrangements we put together be passable?  or is it necessary to get a pro to do them?