I just bought a pair for 1k in Garuda. They are nice, but I have never seen the additional fire damage activate. I know it is random, but is it rare? I have been fighting Hill Lizards in Valkrum; does the fact that they are a fire based mob have anything to do with it?
These are by far the best knuckles for a monk, until you can either afford "Strike Baghnakhs", or wait till you get up to 20 and buy a set of "Cougar Baghnakhs". The Cougar are by far easier to get than the Strike. Strike go for around 30k, whilst cougar go for 6k.
I still haven't found where to get Burning Cesti's other than the AH, and they don't cost more than 6-7k, and well worth every cent er....gil. I used mine up until I hit 20, then went with the Cougar Baghnakhs.
I paid 4k gil for these on Leviathan server. I also have Legionaire's Knuckles which I paid 6k gil for awhile back. I would say the Burning Cesti are quite a bit better with 56 delay less than the Legionaire's Knuckles and a smaller price. The 5 damage fire proc is an added bonus.