The Samsung Gravity Smart comes with a decent 800 MHz processor and is preloaded with Android 2.2. While the 800 MHz processor isn't the fastest available, for an entry level smartphone, it's decent. It has less noticeable lag than the LG Optimus T that's also available through T-Mobile, but it does occassionaly pause here and there. Overall, however, the screen is responsive and tapping keys on the keyboard also gives a decent response.
Call quality was a little bit disappointing for both incoming and outgoing. Incoming calls were, at times, hard to hear, even at full volume. It wasn't due to background noise, rather the calls came in soft even when the caller was talking at a normal volume level. Outgoing calls did have a slight echo and at times, a bit of distortion. When outside, the phone did not filter the wind or the sound of cars very well, causing even more distortion for outgoing calls.
The 1500 mAh battery is the same size battery found on the Samsung Vibrant, T-Mobile's variant of the original Samsung Galaxy S. It has a rated battery life of 5.5 hours for talk time and is supposed to last 15 days on standby. In real world use, the Samsung Gravity Smart lasted a little under 2 days with regular use, including GPS and online browsing.