What effect does head tube angle have on a bicycle? My girlfriend and I are looking at a new bike for her. The one we're looking at has a 70 degree head tube angle while her current bike has a 70.5 degree head tube angle. Unfortunately, none of the local shops has one in stock for her to try out. So my question is:

How much does the head tube angle affect the fit and feel of a bicycle? I'm sure that it has at least some effect over the fit of the bicycle, but how much? And I'm guessing it has an even bigger effect on the steering, but again, how much?
there's a good write-up about this here: http://www.calfeedesign.com/tech-papers/geometry-of-bike-handling/

In general the steeper the angle the more agile the steering. If you like steeper you might say "quicker, more response", if you didn't like steeper you might say "twitchier".

A 1/2 degree difference is probably not going to make that much difference by itself. The other factors, which headset, installation of headset, stem length, handlebar type and setup, etc., will mask the differences here. As the Calfee page mentions, people will adjust very quickly.

Happy Riding!