on a p/kWh basis they’re not actually very good value for money.
They do give of a decent hot heat but the packaging, processing, happy-clappy marketing adds up.
Last year when I ran out of seasoned wood I bought a similar product from a local joiner. He would use an automated hydraulic press to process all his wood waste (kiln dried timber) and the briquettes (c.3.5″ diameter, 4″ long) would drop into a builders agg bag. I paid around £50 for 400kg, which at 5% moisture is around 2.5p /kWh.
However, that link, even if you buy at 1 ton at a time, is costing around 7.5p/kWh
If you buy in 10kG bags, at a time its 20p/kWh
Other things to bear in mind, they do tend to swell when they burn so dont overload the burner. Also they dont dwell as an ember, they burn out so the fire is hard to revive if you have let it die unlike wood embers.
You will find getting hold of cheap willow or poplar, and season it yourself for a year and then even better in a greenhouse for a few months, will give you a hot burn, good for flue gases, and much, much cheaper.
PS I always recommend reading this
http://cosi.co.uk/HeatingAndChimneys/LOGSBioFuelPriceComparisons.html