I'm currently on my second bottle (running the product for the full 50 days which comes out to 2 100 cap bottles back to back). By the second week of use I was getting VERY notable DOMS which would last for quite some time. This was the first indication to me that the product was kicking in. The second most notable and obvious sign that the product was working was that by the 3rd week, my decline bench press went up since starting the product, technically my decline bench press weight went up 5 lbs on the second week but that could have been a weight increase that was going to happen anyway regardless of XF or not, so the fact that it went up another 5 lbs the week after was impressive to me.
The product has continued to improve not only my bench press but all my major lifts in general. Either weights are going up or reps/sets are going up. By far the most impressive natty supplement I've used for strength gains and even on the current second bottle, the effects are still going strong.