I've been using an old rollerball style mouse at my desktop computer for the past three or four weeks, and have found it better than the laser style mouses that replaced them over the last 5 to 10 years. It's a three-button scroll wheel model by General Electric (model 97859).
Here are a few thoughts on the differences:
1. The older ones are heavier, from the ball and the mechanical guts. Even the casing feels strongly built, not flimsy. More importantly, the added weight dampens your hand motion, so that the mouse doesn't take off with a typical movement. A lighter mouse almost has you making an unconscious effort to bring the movement to a stop, like a slight step on the brakes.
2. There's more friction between the mouse and the surface underneath, as the ball runs over the mousepad. This adds to the dampening of motion, like the old school steering wheels that gave the driver tighter control (unlike the newer, looser wheels where you turn too far and need to turn back to compensate).
3. The movement is smooth, fluid, and analog, as the ball spins the wheels in contact with it. It's not noticeably smoother than the laser mouses, but not less smooth either, as lazy thinking about "old technology" would predict.
4. There's more feedback, as a result of the above features. You're just more in-touch with what you're doing, whereas the laser model has a weightless quality that removes the feedback that should be going to your sense of touch. Both models give you visual feedback (the pointer moving on the screen), but the rollerball provides a redundant channel of sensory feedback. It's like using a keyboard with buttons rather than a touchscreen "keyboard".
5. The mouse buttons require a bit more force to press down, and there's a more audible click. They're still simple to use, but that extra bit of resistance and noise provides good feedback when clicking on something. The weightless, mushy, quiet mouse buttons actually require more monitoring on your part because they're so easy to press that they accidentally get pressed more often, and there's no "warning" click that goes off when this accident happens. This is an older vs. newer difference, not necessarily a rollerball vs. laser difference.
6. They're larger and fit more comfortably in the (adult male) hand. Another older vs. newer difference. The small and flimsy types require a somewhat tighter grip whenever your hand is on the mouse, which strains the hand muscles or tendons over time. You can't sense this from only moving it around for a second or two, but that added flexing / gripping adds up over a session of computer use. With a larger mouse, it's more like you're just pushing it around, while your hand rests on top.
The closest analogy I can think of is that rollerball mouses are like paintbrushes, pens, pencils, or crayons going over paper, canvas, or some other rough-ish surface, while the laser type is like the stylus for a touchscreen.
So, they're better functionally and ergonomically. The only downside is their higher maintenance, from the dust that is swept up inside the mouse whenever the ball is rolling. From my level of usage, I've had to clean out the mouse once a week, which takes all of a minute or two. For those who don't remember doing this, you remove the ball cover on the bottom, set the ball aside, and use your fingernail or something small and scrape-y to remove the ring of dust stuck around the two wheels inside. I don't need to clear it out with compressed air or anything extreme.
That downside is by far the most common complaint about them, but from the hysteria you'd think they required laborious scrubbing every couple of hours. Over the course of a week, you spend more time and calories brushing your teeth. But, in our throwaway culture we reject anything that requires even minimal maintenance. Your paintbrush has paint on the bristles and needs to be rinsed off? Nah, fuck it, just toss it out and buy a new brush.
You won't like the rollerball mouse if you prefer the weightless, friction-less experience where ultimately we'll just think of where we want the pointer to go, and it'll go there. If you prefer to keep in closer contact with the tools you're using, give them a try. The difference is not major, but enough to be worth it -- especially considering how cheap they are nowadays. I got mine at a thrift store for a dollar.
Bonus feature: they almost all use the PS/2 connector, freeing up your USB ports for things that you will plug and unplug frequently (not a mouse).