I've been cycling now for over 6 months. I had a very long break over the winter due to a long running cold, but I'm now back into it full swing. I've clocked well over 300 miles on the bike (not that much, but it's quite impressive to me).
I'm *still* exhausted every day by the time I get home. I'm sure I must be fitter but I don't feel it at the end of my journey. Of course I live on a big hill so this is partly to be expected.
I haven't lost any weight from this. This is a bit of a surprise to me - I really thought the weight would drop off (I'm still around 200 pounds). Of course losing weight is a simple equation - calories in > calories out. Or as a co-worker of mine puts it: mouth-hole is bigger than the asshole.
I'm quite tempted to do more cycling (such as a cycle across the US - hah!), and can recommend it - I still make it to work faster than I would in the car (though our offices are moving so that will change real soon now) and I feel like I'm doing my bit for the environment, which gives me a feeling of peace.