I can certainly concur about riding a clunker - my old MTB is just that, and I've only ridden it twice in the four and a bit months since I got the road bike.
The frame was too big for me (not by a great deal, but enough to create problems), it was heavy and it had an annoying habit of chucking the chain off the cassette no matter what I did to the rear mech. On a cold, wet, dark commute, when you have to keep stopping to put the chain back on, it's easy to hate cycling with a passion.
On the other hand, riding a bike that a) fits, b) is light and c) the mechanicals work / play ball makes a real world of difference...