To be honest, both of my R15s are pretty reliable on that front. I think, between the two, I missed 2 shows in the last 6 months. At least from what I was able to tell. There seem to be, based on my reading of other postings, a couple items which causes the R15 reliability to drop. First, if you have lots of show schedule conflicts. I've never had that much of an issue here because I don't have many scheduling conflicts - most of the time either R15 is only recording one show at a time.
Second, it appears that the reliability drops when the box is nearing one of it's limits - number of items in your to do list, number of series links, etc. When one of those limits are approached it seems like the R15 starts having more issues.
Personally, aside from the interesting quirk here and there, I haven't hit any show-stoppers on either R15. The only time I had any real issues with missed recordings happened over the summer while we were on vacation. It's an issue that may have been fixed with one of the subsequent s/w releases. When the R15 loses power, and comes back, it'll try to aquire a satellite signal. If it doesn't find one, it would just sit there... and do nothing until you reset it. While we were on vacation, a couple really nasty thunderstorms came through the area. We apparently lost power, just for a couple minutes, but when it came back, the storm was still running at full strength. So when power came back, there view to the satellite was still blocked because of the storm. My R15 (only had 1 at the time) powered up fine... but because it couldn't aquire the signal, it just sat there doing nothing... for several days until we came home.
Like I said, though, I think this issue was fixed with a subsequent s/w release, but we haven't hit that situation since, so I don't know for sure. And a UPS would have probably prevented that from happening, but again, since I don't have one, I don't know for sure.