Okay, I have been using HMB for about 1.5 months now so I will relate my experience, so far. In the first few days, my legs felt much more refreshed than usual. They didn't have that lingering tired feeling and I was elated at what was happening. Unfortunately, that didn't last very long and I was right back to feeling really stiff and tired after a tough ride like Road to Sky or Ven-Top. What I found, though, seems to mirror Carbonfiberboy's experience. After doing a hard ride like those and getting a couple of rest days in, I am able to do a regular ride at a good pace. My legs may seem tired and a bit stiff but instead of feeling truly "flat" and being slower than normal on the next ride, I am actually able to push a regular pace on a more standard route. Basically, like the OP, I have found an improvement in my performance. In fact, I have since added a routine of squats and dead lifts after one of my rides each week. The HMB doesn't necessarily keep me from feeling tired in between workouts but it does seem to keep me from having a drop off after hitting things hard.
I am now satisfied enough that I will continue to use the HMB after this first supply runs out. I started with the capsules for the convenience but will be switching to the powder for the cost savings since you can get twice as much for less money. I am very curious to see how things are after 6 months when Winter is over and I am back out on the road with the local club.