Used 40D as a backup?

[KameeraPhoto]

I've had a T2i for about a year now. It's treated me very well. I've been interning with a photographer/friend for quite some time. I usually assist on artist/band/musician shoots, but this spring and summer he has a few weddings booked.

I love my T2i, but I've found it to be rather poor when it comes to auto-focus in less than ideal lighting conditions.

I came across a great deal for the Canon 40D used. I'm considering purchasing it with the intent to use it at weddings. It is my understanding that the ISO is not as good (but we'll be using flash heads and flash guns at the wedding, so perhaps that issue should be of no concern to me). However, the 40D has better AF, no?

Could anyone with experience with the 40D relate their impressions to me?

[RichardTaylor]

I have owned a 40D ever since they first came out, and it has about 120,000 clicks on it.

It's a good camera, and it has been problem free.
Easier to use then the rebels (xxxD) mainly because of the big rear wheel.

Havn't used a T2i however the focussing specs read the same.

The lens used has a lot to do with how quick the camera will focus.

Buying used one I would be very careful on how much usage it has had.