I've been playing with the stable version of the client (1.2.2). I wouldn't say it's easier to set up than vent, although it's in some ways a more advanced tool and it tries much harder to get people's mics set up properly.
First, the wizard asks you to open your volume control and play with your record volume. Windows does not make this easy; I was only barely capable of finding the (I *think*) appropriate volume control for my headset. I would expect most Windows users to fail this request and get frustrated.
Second, while the wizard is running it loops audio input to output, and in the process causes ringing feedback which is a big distraction and makes you constantly wonder whether you have something adjusted wrong, even though nothing seems to affect it. This happened much more when I used headset input and speaker output, but also happened even when I used headset input and output. I assume this is due to automatic gain control being applied in the looping--the natural attenuation of speaker/headphone output to mic input was being defeated by the AGC amplification.
Like vent, it can use push-to-talk or audio level to decide whether to transmit. It can also use (and wants to use) signal-to-noise. Unfortunately, again because of the AGC, signal-to-noise recognizes pretty much anything with a signal, even something pretty far away, such as a ringing phone or someone else talking several feet away. Sniffing or heavy breathing also seems to have as much signal as talking.
Setting the push-to-talk key via the wizard is easy enough, but setting it using the basic controls is completely non-discoverable. There's no option to do so in configure -> settings -> audio input; instead, you have to go into configure -> settings -> shortcuts, click to add a shortcut, then choose the push-to-talk function and set the key. It's not hard to re-run the wizard and page around in it during normal operation, but a lot of users aren't going to realize that's an option. (Plus, of course, you get the sound looping with all of its ringing feedback whenever you re-run the wizard.)
The included noises for push-to-talk notification are much more distracting than Vent's short buzzer noises.
Overall, I'd say Mumble suffers from the same usability problems as a lot of open-source projects. Vent is hardly a shining beacon of usability, but most gamers seem to be able to muddle through it. I'm not sure the same could be said of Mumble at this point--although for the people who do, the resulting quality might be better.