The two big problems with DSLR mic inputs have traditionally been the AGC and weak pre-amps that keep the sound bed kind of high and full of hiss.
I made this video to demonstrate the level of audio possible on budget DSLR shooters like the T2i by using a simple hack to enable manual audio control, and using a pre-amplified signal.
Nikkor 50mm f/1.4
Sennheiser G2 wireless lav mic system
Canon Powershot A460 cheapo point-and-shoot used to snap photo of setup for illustration.