AStrong evidence, for the people named belowOther
The short answer
Genuinely effective, and the honest version is still that vegetables, pulses and whole grains do this better and bring everything else with them.
Who this is actually for
People who do not reach the fibre intake they should from food, and people with constipation or irritable bowel syndrome.
What it is legally allowed to claim
Nothing. There is no authorised EU health claim for this substance, so any packet promising a specific benefit is making a claim it is not entitled to make. That is worth knowing before you judge the price.
How much is too much
No upper level. Build the dose up slowly.
Cautions and interactions
Take with plenty of water and separate from medication by at least two hours, as it can slow absorption. Rare choking risk if taken dry.
Ask a pharmacist before you start anything. Interactions with prescribed medicines are the most common way supplements cause harm, and they are easy to check. Bring the actual packet, or a photo of the ingredients panel, because doses and forms vary enormously between products. Nothing here is a substitute for advice about your own situation.