The 40D is a great Camera. I haven't touched mine though since I got a 6D. With the grip it feels like a brick compared to the 6D. In fact without the grip it's still noticeably heavier than the 6D is. It's low light performance is pretty weak though. Most shots I ever took at ISO 1600 I considered to be of too poor quality even for simple web use. I was unhappy even with ISO 800 shots much of the time because of the noise and the overall image quality degradation. At ISO 100 and 200 though it takes great shots. I love the colors it produces in that range and I captured lots of great moments that way with mine.
For someone inexperienced though it has a steeper learning curve. I let a friend borrow mine once and she had a hard time understanding how to use all the controls to get the shots she wanted in changing conditions. Another friend who was taking a photography course had an easier time learning how to use it when she borrowed it for a shoot.
This hasn't been my experience to be honest. You do have to be careful with ISO 800+ in terms of good exposure otherwise there'll be a fair bit of noise. That said I have many great shots at ISO 1600 that are good enough to not be bothered by the noise even at full screen viewing on a 24" monitor.
A little bit of ETTR will help immensely and most of the time you don't have to actively overexpose as the metering system will often overexpose when it is dark enough to need ISO 1600. Pulling back a third to half a stop does great things to the shot as long as highlights aren't clipped