I try TAV from time to time, but I rarely have a use for it for what I tend to shoot. I figure its good for sporting events when you want to control how much motion is shown in a shot, a lot or a little. Maybe lunar photography it can come in handy. But I usually find that when I start in T/AV I end up just going to M and figuring it all out for myself.
Yes, I use T/Av a lot outdoors. I need to control both speed and aperture but leave the ISO in auto from 100-1600 or so. Lets me concentrate on the two main exposure ingredients and not worry about getting the sensitivity right.
I've just started playing with TAV for airshow use,allows me to choose an apt shutter speed AND stop the bigma down.Very usefull with changing light as it allows me to concentrate on the subject (not that i'm any good but hey ho...)
I use it in low light, so I can get the lowest ISO possible while still having a shutter speed I can hand hold, especially for slow lenses. On the DA21, I'll dial in f/3.2 or f/4 at maybe 1/20 sec, and let the camera set the ISO. I hate going back to the exif and find that I'm shooting in ISO 3200 at 1/90.
I am going to start using it alot more. I have found that when shooting street photography and you see someone in the shadows all you want is a clear shot knowing you have a shutter of 1/125 and aperture of 5.6 is all that matters, especially for B&W conversion.
I've found it very useful for motorsport. I can set the shutter speed and aperture I want and let the camera worry about keeping the exposure correct, which is especially helpful in changeable weather.