For me they don't stop calluses, but they do limit them. But what's important to me is that I found I can lift more easily and get more reps in because they make lifting more comfortable. I won't workout without them.
I find that cheap, effective rubber weightlifting grips like these:
do a better job of preventing calluses, plus they keep my hands from sweating, don't bleed leather-stain onto my fingers and palms and grip better than gloves so I don't get the friction calluses I still got from gloves and can lift heavier weight. Thanks to them I have a hard manly body and soft womanly hands.
I use them all the time but they have started to smell pretty bad! I have a running joke with my gym friend that I will put my hand over the face of a trainer I don't like when he hogs the equipment...
I always use gloves, the only exception is when I am doing lat pulldowns, where my grip has to be such that gloves don't hack. I've used a variety of brands. I'm about to throw away the one I've used to date, they are totally worn out and they "bleed" if I do any sort of cardio at all and my hands get black from the moisture inside. I think if you use them, it wise to only do so when you lift or use a lot of weight on the machines. If you sweat a lot, you may have issues with the interior of your gloves.