Call quality was generally good as the M900's earpiece is nice and loud while the mic does a decent job of making speech sound clear and distinct even with a fair degree of noise going on in the background. Battery life, however, isn't so good.
As you would expect, that big screen tends to drain a lot of battery power. We managed to get around a day out of it before it was begging to be plugged into the mains for a top up, which is significantly less than the Touch Pro2, which managed around two days before needing a recharge.
As with all of Acer's current smartphones, the M900 is built around Windows Mobile 6.1, but Acer has added its own interface over the top in an effort to hide away some of WinMo's less desirable interface elements. Called Acer Shell, this presents you with a virtual desktop laid out across three screens with various office items stream around it. Tapping on these items launches corresponding applications or opens up various settings screen. It works well enough and there are some nice touches such as the way the photo frame and rollerdex icons always show the last picture and contact that has been accessed. However, the graphics are a bit boring and the whole system is a long way from the slickness of HTC's TouchFlo interface.
The M900 is not a bad handset but it's let down by its chunky dimensions and less than impressive build quality. However, the biggest issue with this phone is that it just doesn't offer anything that isn't done better on HTC's impressive Touch Pro2. That device is smaller yet has a better keyboard, a sexier design and more sturdy build quality. As a result it's hard to see why anyone would choose the M900 over it.