Fashionably late to the party but here's my 1.75 pence.....
I'm a big fan of rollers but they're not a miracle cure for anything.
Technique first - I'm kinda half-in and half-out over the technique benefits of rollers. If you ride like a Jack Russell humping a leg then you'll know about it on rollers because you'll bounce the bike off. Riding rollers by itself won't improve your technique, it'll just make you more aware of any lack of smoothness in your pedalling action. If you want a better action, then you'll have to concentrate and practice whenever you ride a bike, indoors or out.
Same if you're a bit of a low-cadence grinder. Not a problem in itself if you're banging out the big Watts, but a higher cadence for a given power output can be more efficient and is certainly easier on the knees. It's quite difficult to maintain a low cadence on rollers and there'll be almost no resistance and no benefit, so you naturally tend to spin.
Personally I find the technique, position on bike etc not that different to riding a bike down a gentle slope.
As
@Yellow Saddle says, compared to a turbo the resistance (required power) is low and only increases approximately linearly with wheel speed. In contrast, turbos will typically ramp up resistance as the square or even cube of the wheel speed. You can get rollers with resistance units, but don't. They're expensive and add unnecessay complication.
The limited resistance makes rollers good for recovery-type rides and coming back from injuries, especially knee problems that are exacerbated by anything high-resistance. Once you're reasonably proficient, a steady rollers session will likely sit you somewhere around a moderate aerobic effort, which over time can help with weight management if that's a factor. I'll sit on mine at about 60% FTP and 60% max HR for an hour watching the One Show and burn a big slice of cake.
"Core strength" - in quotes because it's become dogma without really having much meaning. Without blabbing on, I quite like this article ....
https://www.britishcycling.org.uk/k...re-stability-exercises-and-cycling-training-0
Even then, I doubt rollers will do much for functional core strength and robustness. I think some of the myth about this comes from when you first start riding rollers - everything will be tense! EVERYTHING!!! After 10 minutes you'll be knackered and more than likely your back, abs, shoulders and calves will ache to buggery. It'll feel like a weights workout. With a bit of practice everything will relax and your pedalling action will become more natural and 'ride-like'. Before you know it you'll be able to whack out an hour no problems.
And then you'll fall off and hurt yourself. Learning to ride the things falls somewhere between a skill and a knack (a snack?). Do not skip the door-frame apprenticeship - it really is the best way to learn without hurting yourself too much. The big offs come when you get cocky. My best effort was demonstrating an on-bike dismount in about a foot of clear space. I fell into the showoffee's top tube and nearly snapped his bike in two. Just don't!
In terms of kit, there's not a lot to them so there's no need to go top-of-the-range. I think they're more expensive than they should be when compared with turbos. I suppose this is just economies of scale; you can pick up a turbo trainer in Lidl but rollers have a limited market. I haven't looked, but I imagine you could find quite a few second hand on scambay.
Unless you've got a dedicated turbo bike, rollers are much easier to set up - grab your nearest bike (subject to geometry), plonk it on and away you go.
Finally, and this is purely personal, there's something emotionally more satisfying about riding rollers over a turbo session. Even when you're totally comfortable, you have to leave a bit of your brain engaged with staying on. I tend to let my work problems follow me home, so something that forces me to concentrate on a purely physical activity and gets the endorphins up really helps me sleep at night.
I like rollers
