I really like Onyx sand, it's about the easiest to plant stems in and does not lock like Flourite, has a nice single color and growth is excellent. A few feel it's better than Flourite.
I've used it for many years. Since then, I've mainly switched to the ADA aqa soil as the sole substrate. Aqua essentials sells it in the UK, and the other Onyx from SeaChem.
For harder water plants, the Onyx has no other to compare to, whereas the ADA AS is great for the acid ranges, and flourite is a neutral substrate that is good for folks that uproot and move things around a lot.
Plain sand works, as does sand+ soil, but these designer substrates make things easier and nicer over time and are generally worthwhile iterms.
Regards,
Tom Barr