Unreliable at best, useless at worst. I used stability with my first fishless cycle. Used it as described on the label for a week, then after seeing a thread here, dumped the rest of the bottle in. The cycle still took five weeks, which isn't short even without adding bacteria. Bio-Spira (I've heard the UK equivalent is Bactinettes) has the potential to work as advertised, but they're sensitive. If they're mishandled in shipping, at the store, or on the drive home, you're SOL. I wouldn't trust any of them to replace a cycle, but they might help as a supplement to one.
I saw a post the other day about some of the products containing bacteria that do live in a tank, but aren't the ones responsible for processing ammonia and nitrite. I'll see if I can turn it up again.
