Auto-focus on digital SLRs.

I am new to the auto-focus world, both my film cameras are manual focus.
I just got my hands on my sister's Digital Rebel XT (DRXT) to just learn 
some basics of the camera and see what features I really like.

One thing I am developing a disliking for is auto-focus.
On the DRXT it sometimes goes into fits zooming in and out then refuses 
to snap a picture. I find the auto-focus slow and a pain in the ass.

I understand that the speed of auto-focus can be a function of the lens 
and the effectiveness on the amount of light (if I understand this 
correctly)

Other than things like numbers of points and speed of lenses I don't see 
much on the effectiveness of auto-focus. 

How do the hi/low end cameras differ for canon?
Is it better/worse with other brands?

If this is representative of the other cameras as well I think I may add 
weight to the quality of manual-focus for my future purchase and largely 
ignore auto-focus.

Thanks,

Steve