I can echo some of what Doohickie says. my commute is 13 miles each way, which lets me brag that I "do a marathon daily" but...

- it takes about an hour, a bit longer on the way home. two hours is a lot of time on a bike. great exercise, but it's a lot of time. I'd rather be biking than driving, but if I take the commuter rail I could get some work done... it's just a long time. longer in the winter with all the gear
- heat-wise I'm OK for about the first half hour and then my toes freeze. only solution I've found is the chemical toe warmers, which aren't cheap.
- although the ride is exhilarating when it's warm (i.e., above freezing), in winter I find the ride exhausting

I sure wish my commute were more like 8 miles. I'd think twice before doing this again.